The one for Palpitoad is obvious. Its maximum Speed stat at Lv. 100 is 260. It has the ability Swift Swim, which doubles your Speed in rain. Multiply 260 by two, and you get 520. Then give it a Choice Scarf to hold, and you have 780 Speed. It's faster than a Speed Deoxys (That hasn't used a move like Agility).
Palpitoad@Choice Scarf
Nature:Jolly (-Sp.atk +Speed)
EVs: 252 Attack, 252 Speed
Rain Dance
Waterfall
Whatever you want
Whatever you want
EDIT: Have another Pokemon use Rain Dance. Palpitoad also can't learn Waterfall :(. Just outspeed everything and make the most of it ;)
And, with Waterfall you can make targets flinch a lot.
For Cofagrigus, try this.
Cofagrigus@Life Orb
Nature: Modest (-Atk +Sp.atk)
Evs don't matter.
Mean Look
Will-O Wisp/Toxic
Hex
Calm Mind
Calm Mind boosts Sp.atk and Sp.Def. Get a stat boost, then use Mean Look to prevent escaping. Once you inflict a status problem on the opponent, they'll be petrified. Then use Hex for STAB and 100 base power.
Banette can be a Trick Room sweeper with the nature Brave (-Speed +Attack). It can have some STAB moves such as Shadow Claw.