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Explosion has the highest base power of any move, 250. Also, because the move halves your opponents defense before it deals damage, it has an effective attack power of 500.
The most damaging moves that don't involve suicide are Hyper Beam, Blast Burn, Frenzy Plant, and Hydro Cannon with 150 base power, but they require you to miss a turn to recharge afterward. Overheat and Frenzy Plant have 140 base power but lower your special attack two stages after use. Hydro Pump, Blizzard, Fire Blast, and Thunder have 120 but only 70% accuracy. Zap Cannon is also 120 and always paralyzes but only has 50% accuracy.
So yeah, they're powerful, but they all have some drawbacks.