This answer used to contain a very different team. Then I did some research on Smogon and Speedrun.com and decided that my old team sucked. Several people commented on my old team, which is why those comments don't make sense now. I still have not yet played BW at all, so I don't understand how I got so many up-votes.
Stoutland
Ability: vital spirit -> intimidate
EVs: 100 Spe
- return
- crunch
- dig
- work up
Availability: Early-game (Route 1 from levels 2-4 at a 50% encounter rate).
Typing: The line's members are Normal-types and neutral against everything save Shauntal, whose Ghost-types are immune, and Marshal, who hits the line super effectively.
Stats: The Lillipup line has solid stats except for Special Attack, with Stoutland having 100 Attack, 80 Speed and 85/90/90 bulk.
Movepool: Tackle and Bite carry Lillipup well until Take Down at level 15 and (as a Herdier) Crunch at level 24. Return via TM at Nimbasa City is the line's best STAB attack once they have high friendship, and the Work Up TM can be useful to boost offensive stats.
Major Battles: The Lillipup line has a solid showing in all major battles, as few opponents resist Normal, and Ghost- and the rare Steel-types are handled by Crunch and Dig. Work Up can help the line sweep some fights from Elesa onward.
Additional Comments: Lillipup is consistently a great Pokémon for Gym Leaders but is too reliant on Work Up boosts to do its job at the Pokémon League. Get the Vital Spirit ability as Lillipup, as it turns into Intimidate as a Herdier onward, letting the line take physical hits better.
I chose Stoutland mostly because the current world record speedrun used it. The speedrun used reversal and did not use intimidate, dig, or carbos. However, I think most BW players don't want to keep Stoutland's HP low just to maximize reversal's power, and they don't care very much about the extra time it takes to use intimidate or dig or get the 10 carbos.
Simipour
EVs: 100 Spe
- surf
- work up
- acrobatics
- blizzard
Availability: Early-game (Dreamyard (Snivy) / Pinwheel Forrest (Inner) rustling Grass at 10%).
Typing: Water typing is good for most Gyms aside from Drayden/Iris, being effective against Clay and neutral elsewhere.
Stats: The monkeys have all-around good stats, most notably 98 offenses and 101 Speed.
Movepool: Water Gun becomes the fantastic Scald at level 22. Simipour gets Dig, Acrobatics, Shadow Claw, Rock Tomb, Rock Slide, and all Fighting-type TMs for wide coverage and Work Up for setting up. Scald later upgrades to Surf, and Blizzard is bought at Icirrus City.
Major Battles: Simipour can hit Burgh's Dwebble, Shauntal's Chandelure and Golurk, and Grimsley's Krookodile with STAB attacks. TM coverage handles almost everything else.
Additional Comments: Panpour's Water typing and wide coverage allow it to beat most Gym Leaders, but it is still reliant on Work Up boosts for the Pokémon League. Evolve at level 22 after getting a Water Stone in Castelia City.
Simipour is the special attacker of this team. Its water attacks are useful against several ground and rock Pokemon that are not weak to this team's other attacks, as well as fire Pokemon before obtaining the TM for dig. I chose this over Samurott because the Dreamyard monkey is guaranteed to have the advantage over Cheren's starter and the first gym. Acrobatics and blizzard are powerful attacks that cover grass and dragon and are not learned by any other team members.
Darmanitan
EVs: 100 Spe
- fire punch/flare blitz
- belly drum
- rock slide
- dig
Availability: Early-game (40% chance to appear in Route 4).
Typing: Save for Drayden/Iris, Fire hits all Gym Leaders and Elite Four members for at least neutral damage and is hit super effectively only by Clay.
Stats: Darumaka is decently fast, and its high Attack buffed up by Hustle allows it to hit every foe hard; its shaky bulk is fixed by Eviolite. As a Darmanitan, it hits even harder, is way faster, and has enough bulk to take neutral hits well and even avoid OHKOs from super effective moves.
Movepool: It learns Fire Punch at level 22, Belly Drum (which it can safely set up with as a Darmanitan) at level 30, and Flare Blitz at level 33. Hammer Arm comes upon evolution, and Superpower is learned at level 47. TM-wise, it can be taught Brick Break as an alternative to Superpower, Rock Slide, and Dig, the latter of which is good for Shauntal and Ghetsis's Fire-resistant Pokémon.
Major Battles: As a Darumaka, it only ever struggles against Clay. Burgh and Elesa lose to Darumaka, although it needs Eviolite for both. As a Darmanitan, it sweeps all the other Gym Leaders, with Drayden/Iris falling to Belly Drum. At the Elite Four, it can use Belly Drum strategies again to sweep all but Marshal. It is useful against N and Ghetsis, the latter being swept if you use Substitute and X Speed in conjuction with Belly Drum.
Additional Comments: Although Hustle might be annoying, most of the misses are not fatal; it does not prevent Darumaka from being one of the best choices for an efficient run of the games.
Darmanitan has insane stats and learns fire punch and flare blitz insanely early. Rock slide and dig cover fire and rock, and I chose them over Darmanitan's fighting attacks because the team already has a fighting Pokemon.
Scrafty
Ability: moxie
EVs: 100 Spe
- crunch
- high jump kick
- work up
- rock slide
Availability: Early-game (20% chance to appear in Route 4).
Typing: Although it struggles with Skyla, Scraggy's typing allows it to beat Brycen and all of the Elite Four members barring Marshal.
Stats: Scraggy has good Attack and defensive stats, which can be buffed by Eviolite. Its Speed will eventually cause it problems as a Scrafty, but you should have Speed EVs to outspeed some slower threats.
Movepool: Its only STAB move is Faint Attack until it learns Brick Break at level 20. It can be taught Payback at level 23 to take advantage of its low Speed. High Jump Kick at level 31 and Crunch at level 38 are its strongest STAB moves. TM-wise, it can be taught Work Up and Rock Slide.
Major Battles: Excepting Burgh's Leavanny and Skyla, Scraggy does well against every Gym Leader, although it needs Eviolite for all of them as a Scraggy. It also does well against every Elite Four member bar Marshal and is useful against N and Ghetsis.
Additional Comments: The combination of a strong movepool and good typing that threatens a lot of major opponents makes Scraggy a very good choice for a run of the games. Always use one with Moxie over Shed Skin.
Scrafty's stats aren't the best, but its offensive typing and ability are both amazing. Additionally, Scrafty learns powerful STAB moves very early. You can use brick break if you really want, but I think high jump kick's 130 base power is more than worth the 10% chance to miss and take crash damage. Rock slide covers flying and bug.
Drilbur
- cut
- strength
This is the first HM slave. I don't think you can get a Drilbur before the Dreamyard, so you'll need another cut user (such as Patrat) for that.
Ducklett
- fly
- surf
- dive
This is the second HM slave. Ducklett is by far the most common fly user that can also learn other HM moves.
That was my new and improved team. I hope I did my research right this time and I don't have to rebuild this entire team again 4 years later.