I'll add my opinion so;
For starters there are only two 'true' signature moves - a move that is exclusive to only one Pokemon or it's line. Those two moves are Chatter and Sketch, only learnt by Chatot and Smeargle. So technically Chatter is the strongest? :3
Anyway so the strongest in base power is Roar of Time, Dialga's Signiature Move at base 150 power (Remember it has the recharge round though).
In terms of overall, I don't really think there is one. There are alot of powerful moves that contend, like Yveltal's Oblivion Wing, which not as powerful restores a large amount of the damage taken to Yveltal's HP, Darkrai's Dark Void which puts opponents to sleep, Arceus's Judgement which is a strong base 100 and always gets STAB, King's Shield and Spiky Shield (Aegislash and Chesnaught) which work in a similar way to protect, by protecting against offensive attacks and causing secondary side effects on top of that, and so forth. So yeah really opinionated.