I've never played BW and I might never play them in the future, but I worked hard on this team and I thought it would be a waste of my time if I didn't share it. The whole team can be assembled before defeating Clay, and has a super effective STAB attack against every major opponent except Burgh's Dwebble. The only problem with this team is that it requires at least two HM slaves. Please note that I didn't plan for post-national Pokedex opponents.
-tackle -> arm thrust -> hammer arm
-ember -> flame charge -> heat crash -> flare blitz
Notable NPC battles against fire-weak Pokemon: Bianca, Burgh, Clay (again, don't use Pignite against Clay), Brycen, Bianca, Grimsley, N (I'd suggest you not use Emboar against Carracosta because it knows waterfall), Ghetsis
Notable NPC battles against fighting-weak Pokemon: Cress, Cheren, Lenora, Charles, Clay (I'd suggest you not use Pignite against Clay because his Pokemon know bulldoze), Cheren, Brycen, Bianca, Cheren, Grimsley (I'd suggest you not use Emboar against Krookodile because it knows earthquake), N, Ghetsis
As far as I know, this is the only way to get any fighting attacks before defeating Cress, who uses a Lillipup. Because of this, if you want to use fighting attacks against Lenora, you need to evolve Tepig as soon as possible.
-vine whip -> seed bomb
-rock tomb -> rock slide
Notable NPC battles against grass-weak Pokemon: Cheren, Cress, Cheren, Charles, Clay, Cheren, Bianca, Cheren, Shauntal, Grimsley, N, Ghetsis
You can get Pansage before the second battle against Cheren.
-bite -> crunch
-sand tomb -> dig -> earthquake
-mud-slap -> strength
-rock tomb -> rock slide
Notable NPC battles against dark-weak Pokemon: Skyla, Bianca, Shauntal (I'd suggest you not use Krookodile against Shauntal's Jellicent because it knows surf), Caitlin, Ghetsis
Notable NPC battles against ground-weak Pokemon: Elesa, Charles, Clay, Cheren, Shauntal, Grimsley, N (I'd suggest you not use Krookodile against N's Carracosta because it knows waterfall), Ghetsis
You can get Sandile before defeating Elesa. Unlike all other ground Pokemon in the whole game, Krookodile gets STAB for crunch, which lets it land even more super effective attacks than it already does.
-spark -> volt switch -> thunderbolt
-pursuit -> light screen
-double team -> encore -> roost
Notable NPC battles against electric-weak Pokemon: Charles, Skyla, Cheren, Bianca, Cheren, Shauntal, Caitlin, N (I'd suggest you not use Emolga against N's Archeops because it knows stone edge)
Notable NPC battles against flying-weak Pokemon: Bianca, Grimsley, Marshall
Of all the electric Pokemon in Unova, Emolga comes with the best second type and STAB acrobatics. You can also teach it roost.
Ability: ice body
-avalanche -> icy wind -> ice beam
-taunt -> mirror shot -> flash cannon
Notable NPC battles against ice-weak Pokemon: Clay, Skyla, Cheren, Bianca, Drayden/Iris, Cheren, Shauntal, Grimsley, Caitlin, N (I'd suggest you not use Vanilluxe against N's Archeops because it know stone edge), Ghetsis
This is the last member of the team. Vanilluxe is faster than Beartic, has better defenses than Beartic, can be obtained earlier than Beartic, and has a much less stupid design than Beartic.
Good HM slaves to go with this team include Basculin (cut, waterfall) and Ducklett (fly, surf). Neither can learn strength, so I suggest teaching strength to Krookodile.