In terms of base power, the most powerful move is explosion, then self destruct, but they both cause you to faint after using them.
Frenzy Plant
Psystrike
The most powerful rock type move is either rock wrecker and head smash
According to Pokemon Black and White, its most powerful move is Double-Edge.
Avalanche or Blizzard
I heard it is shadow force.
I have one, Its Night Burst, It will learn it.
Sheer cold its a 1 hit KO move.
meteor mash and iron tail
its called " guard share "
The most powerful move is frenzy plant. Only the grass type starters of each region can learn it from a guy on route 228.
If it was a move it learned by leveling up, it's most likely Close Combat.