Here are some powerful moves with no bad effect:
Eruption/Water Spout: Base power 150
Gyro Ball can have a base power max of 150.
Trump Card can have a max base power of 200.
Magnitude can have a base power of 150.
Punishment can have base power up to 200!
Reversal/Flail can have base power up to 200 also!
Spit Up can have base power up to 300!
These are some of the most powerful moves in the game, although to use them correctly you must know a bit about how they do damage.