Lapras always catches me off guard. You'll run into once in maybe 100 battles and it seriously takes me by surprise every time. Its even cleanly sweep me before, definitely an underrated threat. Its also has just the right amount of bulk, living something that I'd definitely thought would KO and then recover 100%.
On a similar note a 'mon I've been tinkering with is Offensive Gorebyss.
Gorebyss @ White Herb
Trait: Hydration
EVs: 56 HP / 252 SAtk / 200 Spd
Modest nature (+Spd, -Atk)
- Shell Smash
- Rest
- Surf
- Ice Beam / Baton Pass
The idea is that instead of just being a one trick pony(smash and pass) Gorebyss can actually pose a serious threat to rival teams. Now for the full offensive groove you'd run Ice Beam for coverage but it is seriously hard to not use Baton Pass with this thing, especially when you're running screens(like me). Common rain stall teams tend to just fold in against thing thing once water immunes are gone / Dragonite. The key, as with Lapras, is that a free, no draw back Rest is amazing.
Anyway this guy is really cool and has been surprising a lot of my opponents recently. Be warned though, even after +2 Gorebyss is still outspeed by many common scarfers.