Magic Guard is my favourite ability, because its wounderful against stall and maked reuniclus to a top ou pokemon.
Moxie, because of getting that boost from death fodders or Sucker Punch, etc. or.. Shed Skin, because it lets you shed of any status, y'know?
From a non-competitive standpoint and from the kidder inside of my I like simple the best. If you look at a bidoof and say that it's anything but you're crazy. I also like using simple calm mind swoobat in NU from time to time. But on a more serious note I like justified the best, making them terrakions faster is always a bonus.
Poison Heal, totally awesome. Intimidate is pretty awesome too, though, and when it first came around it was extremely popular.
I feel that Filter is the most under-appreciated ability just because it has bad distribution. But my favorite ability is probably Tinted Lens. Making basically all your attacks neutral would be great if anything strong got it. I'm also surprised at the lack of love for Huge/Pure Power. :0
I'll come back with a new favorite: Shed Skin. Being able to set up on walls without giving a f*ck about status is pretty amazing. My favorite moment in Middle Cup was switching Dragonair on a Roselia with T-Spikes on the field, getting poisoned so it couldn't Sleep me. Set up Dragon Dance, after which it immediately shed poison and swept. Lol. Badass dragons at their finest.
Pick up or wonder guard Pick up because you can get rare candy and helpful stuff Wonder guard because it's like hax
Sand stream :3, Dat sp def boost. otherwise its type immunites like water/volt absorb,flash fire, etc.
Moody Broken shit was fun to abuse. STORED POWER SMEARGLE Made Bidoof Uber :3 Illusion Pair this up with the right poke and gg.
Competitively, Regenerator takes Slowbro from a good physical wall to one of the best Pokes in UU. Generally, though, I think Anger Point is really awesome. Never seen it work, but if it did... O_o. Sadly, most of the Pokes who get it are pretty frail and probably wouldn't survive a crit :L
It would be useless though; Guts increases Attack and Starmie is not a physical sweeper. Couldn't help but point that out.
Prankster by far is my favorite ability--Stun Spore from Whimsicott before opponents set up, etc. Prankster Copycat Roar Riolu wins. Just saying.
Anger Point. >If only all my pokes had anger point, nobody would dare to hax any more. But honestly speaking, Best ability ever is wonder guard. Except its user has.... quite a few weaknesses. Plus 1 HP.
Abilities such as Water Absorb, Flash Fire, Sap Sipper etc etc. I like having abilities which not only give immunities but boosts health or a stat. Makes team synergy even better, so yeah..
I like heatproof just for the sheer fact that most people expect a levitate bronzong and when their fire attacks do so little to him, they keep on going XD