>>14561208I love him. Because he's tanky, you get to see the effect of Contrary a lot more often than you think. And his curious typing doesn't leave him weak to very much at all (Dark/Psychic), so he's a reliable switch-in to damn near everything. He learns Psybeam pretty early, and Psywave is also good because it does set damage, he gets Payback and Foul Play (or whatever Dark move it is that uses the opponent's attack stat) so you can absolutely WRECK Strength-based Pokemon in no-time flat. I would keep his fourth slot open for any status moves you want (Topsy-Turvy is great for anyone who tries to set up, but since the AI isn't that smart most of the time, I swapped it out for Hypnosis).
Just remember, BEFORE Inkay levels up at 30 (as in, before you get the notification), turn your 3DS upside-down. It has to be before, not during, or you'll miss your chance. And just hold it like that until the evolution triggers.