I used these two pokemon for a while, and i found them to be both useful and annoying.
Porygon2 @ Leftovers
Trait: Trace
EVs: 252 HP / 128 Def / 128 SDef
Bold Nature (+Def, -Atk)
- Toxic
- Protect
- Conversion 2
- Recover
Designed to stall and be extremely annoying, it's large defenses make it hard to take down, and when it's type keeps changing, it's hard to deal with. clever people will try to set up on it, but with Encore Whimsicott - it becomes less of an issue with good prediction.
Gengar (M) @ Black Sludge
Trait: Levitate
EVs: 4 HP / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
- Taunt
- Disable
- Substitute
- Shadow Ball
Because of his 3 immunities, and many resistances, this works surprisingly well. People rarely carry 2 moves of the same type, and with all the choiced pokes around, sub disable is awesome. Shadow ball gives it some form of attack with reliable STAB. And taunt is to stop things setting up on you when it cant hit you. This also works remarkably well on things like toxicroak and conkledurr, which only has 1 way of hurting Gengar.
Admittedly, Steel types stop both of these working to an extent. but just throwing ideas out.