It really depends on the character but if you ask me, it's the Super Saiyan 3 Kamehameha. All that power combined makes an extremely strong attack. But close behind is the attack (not sure what it's named) Majin Vegeta used against Majin Buu in the episode, "Vegeta's Atonement".
Final Flash!
Actually it is buus planet burst the energy he collected from planets that he destroyed. it doesn't look that strong because Vegeto kicked it away.
in DBGT it would be Omega Shenrons Minus Energy ball.
Goku has many strong attacks but one of his most strongest attacks is when he is in ss3 called dragon fist.
falcon punch
gohan no it is broly ask the question "who is the most powerful saiyan"
the most powerful physic move is Psycho Boost
The most powerful normal type move is Giga Impact
The most powerful rock type move is either rock wrecker and head smash
insidious
Psychic.
In terms of base power, the most powerful move is explosion, then self destruct, but they both cause you to faint after using them.
hydro pump
Magma Storm
Its eruption as the first powerful is blast burn.
yes you can
Frenzy Plant