What do you think is the best? Obviously different abilities have different categories of use (support, escape, dps, etc) but some are just better than others either way. Ill throw out some options but feel free to add to the list.
Gate (Twisted Fate). I think this is the obvious choice. Total map control is hard to beat.
Taunt (Rammus). I could make a case for this being the best tank ability easily.
Deceive (Shaco). Probably the best escape ability in the game.
Black Shield (Morgana). Absorbing damage is nice, preventing status effects is huge.
Time Bomb. (Zilean) Great for harassing, KBs, damage. Dont know that I would put in my top 5 though
Blinding Dart (Teemo). Completely shuts down a physical dps for a bit.
Stealth (Twitch/Eve). Just putting this one out there in general.
Pyromania (Annie Passive). Stun every 5 casts? Crazy good.
Parrrrley (Gangplank). Possibly the best single target DPS spell in the game. Low Cooldown, great damage, cheap.
Defile. (Karthus) One of the best aoe damage spells in the game. Pwns team fights
Rocket Grab (Blitzcrank). So good. A good rocket grab can win games.
My personal pick would be Deceive. I cant think of another ability that is so good at everything (attack, and defense.) Flash all by itself is one of the best abilities there is, and so is any form of stealth. It is also the only instant stealth in the game. That by itself makes it incredibly good, add in the auto-crit damage, relatively low cooldown and mana cost, and you have a pretty much perfect all-around ability.
I could make another argument for gate and thats probably the correct answer. But without PaC and destiny to back it up it is not nearly as lethal. Plus, we all know that TF is pretty OP right now anyways.