So what do you think..

gL4C1eR

..is the best ability in the game?

I personally feel it's Unstoppable, currently Epic Deadpool is Immortal because of it. It's like the cherry on top when it comes to tough nodes.

Agent_X_zzz

Concussion and indestructible are some of the best.

TheBair123

The best ability that a champ has? Probably Archangel's Neurotoxins.
The best possible ability? Either permanent indestructible or a permanent 10,000% fury

Sungj

Phasing

odishika123

Perfect parries

Masterpuff

One comes to mind: Ronins stances.

Realistically, indestructible, concussion, and phase.

KRoNX1

Perfect block . Like in horseman gambit .

Nojokejaym

The ability to not gain buffs but still receive the benefits of buffs.

Headroller

Mr sinisters L1 when running suicides.

Nothing better then putting all that hurt ( at an increased damage from what you get dealt) on the opponent. Then parrying and pausing all that hurt for longer.