Through playing (and experience from Pokemon Rumble on the Wii) I have known of the types of Pokemon that everyone should have to have great fun with this wonderful game. I will also go through the means to get such Pokemon.
In this game there are a few types of Pokemon (with many other variations and sub-categories) that make doing what you want almost effortless- whether you want to recruit a certain Pokemon (Gutsys) or search for legendaries or anything else for that matter (speedys). The previous two are the main two, after that it really comes down to your preference for others (like punches (or pickys if you only need one move))
I will go over Gutsys first, as they are the keystone to all others. A Gutsy can be anything with the following attributes-it must be Gutsy (really only gutsy+ or ++ though), and must learn focus energy.
Now I will go over how to acquire such Pokés. It does take time but it has proven tried and true by yours truly.
STEP 1- From scratch, assuming you have nothing. That's ok, you don't need anything for this first step. First of all if you happen to have a Poké with gutsy from your travels already that's great and use that in this step initially. If not that's absolutely fine and use any old Pokemon (preferably a powerful one). With the poké you have look for and defeat in hopes of tipping any poké that can be a Gutsy (I like remoraid so I'll use it as an example). So say you befriended a gutsy remoraid after searching for a while, Awesome! Step one complete.
STEP 2- Get it focus energy. Hopefully you have enough pokés with focus energy from your travels thus far but if not don't worry! You have two options here. One is to defeat in hopes of tipping pokés that can know it and when you have enough go to the move school and teach your remoraid. Option two is to use one of the move machines that grant moves up to one star and wait for focus energy to appear. Also get another move, preferably one that is quick and not very powerful. Once step two is complete...
STEP 3- Once you have your Poké with gutsy and focus energy (in this case my remoraid) look for the same poke you have but with the rusty trait. this will be significantly easier since you have your gutsy poke with focus energy as opposed to nothing special. Once you have found it return to town and switch to your fastest poke (to save time). Now use the rusty reset exploit (go to the fountain and restore your pokes then check the trait of the previously rusty poke and if it isn't the one you want just SR (Soft Reset) the game and keep returning to the fountain to change it at will!) to get it Gutsy+. Now let it learn focus energy and a fast, weak move and you have your first true gutsy!!!
With this basic setup it is far easier to recruit which ever pokés you want, but there is an even better way. Over the years (yes, I have actually played this game for that long already) I have experimented with different moves to inflict a critical hit on the desired Pokémon (and immediately after switch to something that will take it out swiftly earning me the Poké), and I have found that one combination really trumps all I have tried before it. Gutsy+ Remoraid with focus energy and bullet seed. That and Gutsy+ nidorino (nidoking) with focus energy and beat up.
These two combinations work the best for me because of my preferred style of play but something different may work for you and that's fine! I use Remoraid the most by far because it's usually better to stay a fair distance away from bosses. It should be noted that before I employ my gutsy stratagies I always weaken the boss to around 1/5 to 1/6 health first. I stay just outside of the bosses attack range (using focus energy before the last boss attack), then, after they have used their last attack before blowing smoke I go behind them (easier to critical hit from behind) and fire bullet seed away until it critical hits then switch to the best suited poke to take it out then do just that as quickly as possible, befriending me the poke every time. With nidorino/king I stay away until the last attack then move in close behind it and wail until it critical hits.
I like bullet seed and beat up especially on gutsys because they can be very quick attacks (faster if the poke you are hitting is just within attack range) and because they din't inflict much damage. Something I didn't mention above is that if the target poke looses enough health before a critical hit is landed I switch to somethig that knows heal pulse (latios/as for me) then heal the boss (to around half health) then weaken it again (usually to 1/5 or 1/6 health) then start over with the gutsy.
Exactly why it is better to inflict as little damage as possible to the Pokémon desired to be tipped is because you don't want to possibly kill it and not get it because you killed it without landing a critical hit on it first. One way to do this very well is to go into legacy mode and get a Gutsy that has a very low power level so it doesent inflict almost any damage to the desired Pokémon. the obvious downside to this is it could get KOed instantly from just about any attack thrown at it so be careful!
So after you get your first gutsy you can EASILY befrend ANY poke you want!!!!! (keeping in mind BOSS legendaries are pushovers (as in they are always tipped anyway). NOT the legendaries that rarely appear on certain levels though)
Now Speedys! With all this gutsy knowledge above it now makes sense to not waste any time going through searching for the poke you want with a slow/any old poke. that takes FOR EVER and I don't know about you but I don't like to waste time. Luckily it is VERY easy to get a good speedy with the at least one (I have many, but usually use 4 gutsy+ Remoraids) Gutsy obtained from the methods above!
As you may know Pokemon are tied to their base stats (and move pools) from Pokemon Black and White. With this knowledge, looking at the top of the speed chart we find Speed Forme Deoxys at the top, which is quite understandable but harder to obtain than two comparably fast pokemon. Jolteon and Ninjask are the two most practical speedys to go for, Jolteon more so than ninjask strictly because of rarity. Ninjask is, however, slightly faster due to it being a hovering/flying catagory of pokemon as opposed to the walking/runnung catagory.
Any pokemon with considerably high base speed and that can learn a speed increasing move (moves that increase "evasion" increase speed, as I assume evasion is speed in this game, but by around half so I don't count those moves) can become a speedy but I really only focus on Jolteon and ninjask. I will now lay out the incredibly simple steps to obtain a Speedy.
(All steps below assume you use a gutsy.)
STEP 1- search for the speedy you want (Jolteon in this case) that is rusty.
STEP 2- Use rusty reset to get speedy+
STEP 3- let it learn a speed increasing move (agility for jolteon) and you have your Speedy!!!
Now you can get ANY pokemon you want WAY faster than before!!!!! I dare any Poke fan to say that this game isn't fun after they have played for a little bit with a good Gutsy and Speedy alone!!!!!
And just when you thought this game couldn't get more fun there are the best pokemon for attacking (punchys and pickys (and others))!!!!! I hesitate to group them all under one, or two names but it is the best I can do right now in terms of categorization. These are the pokemon that hit the ABSOLUTE HARDEST!!!!! And are incredibly fun to use! There are many pokes that fall under this category and I will go over the most simple ones now. looking at the Attack AND SP. Attack charts for BW (both because attack in this game is both the Attack and SP. Attack stats added together) it is seen that Attack forme Deoxys tops both of these charts not surprisingly.
There are also others that have high numbers in both stats (mainly legendaries but some non-legends as well). To be honest I haven't studied the relationship between these charts as much as I have done selective experiments on specific pokemon, moves, and abilities. So, the pokes that have high numbers in both Attack and SP. Attack will have the most base attack compared to all other Pokes, and with the Punchy (+ or ++ mainly) and an attack increasing move like swords dance and with the other move slot filled with the most powerful move the poke can learn that is the same type as it to earn the Same Type Attack Bonus (STAB) and if the move is super effective to the opposing poke then, well. you have the absolute hardest hitting poke (strictly counting attack power)!!!!! Along with this the move that increases attack the absolute most is Belly Drum (even though you loose half of your HP) it is quite godly. Perhaps if a poke that could learn it was found with a blue ability that includes punchy and healthy it could use it multiple times without dieing!!
Something else similar to this setup to consider is Picky. It combines Punchy, Teche, and Snappy (the + and ++ variants count towards all three abilities as well!!!!!)! The only downside is that it can only learn one move, but that one move will be faster, and if it doesn't have many stars will be boosted to the power of around a five star move!
Another setup that is different from the first two in that it doesn't necessary include punchy or an attack increasing move. Rather it prevents the reduction of the attack stat that some moves cause. It involves the use of the Steady ability on such pokemon as Rayquaza (the best Poke for this setup as it has the highest attack and SP. Attack stats put together that can learn a powerful move that would reduce its attack) That know moves such as Draco Meteor and outrage.
Needless to say it cannot benefit from an attack increasing move because attack cannot be raised. With the steady ability it is impossible to raise or lower the stats of that poke so it can use those moves without its attack decreasing which is brutal!! This setup can be achieved with any poke learning such moves that would reduce attack, and also interestingly enough substitute (you don't loose HP when creating a substitute, which is just plain fun!). It also works with moves like hammer arm (which would reduce speed).
There are other things that work well like charge + an electric move and others but I won't go into every single one. Besides, it is up to you which you like/want to search for. Additionally I have not tested which ability affects Attack (damage dealt) the most of the abilities that affect attack (such as boomer and grappler) but I will Update this page after I have done so.
There are also other optimum setups to increase HP and defense and possibly poison damage and regenerating HP. One for HP, dare I say the best, is a brawny (+ or ++) blissy. I have a brawny + chansey of power lever 2659 with 3279 HP!! one for defense (the best as well) is hardy (+ or ++) Shuckle with Withdraw, that would be intense! I have not properly researched optimal setups for poison damage or regeneration but when I do I will make sure to update the page with them.
Well That's all for now! I wish more people would get into this truly great game so I can compare my results with others on the main stream. I will also add more when I can.