Who needs viability when you have shenanigans. Lets hear your favorite gimmick sets. Here's a few of mine.

Moody entrainment Smeargle
Give it all the usual stuff plus entrainment. Use entrainment on your doubles partner and claim your victory was all skill. If you have huge balls use it in singles.

Evolite Shelmet
Sounds stupid? It kinda is but as a pure psychical wall it doesn't do half bad. Has moves like recover, rest/hydration, acid armor, baton pass, encore and even spikes. I will always remember the day he stalled out a garchomp. A true dragon slaying knight is there ever was.

Shadow Tag Trick Gothitelle
Not really the most obscure set but I've never seen anyone else but me use it so it's worth a mention. Just throw a choice scarf on this baby and enjoy the next 10 seconds of waiting as your opponent realizes he can't switch. outside of revenge killing you can even cripple defensive or utility mons with trick and kill then at your leisure.