If you're going for powerups for Gatherer, you should get this while unlocking your second powerup of the week. At 180 games to fully unlock two powerups, you need to clear an average of 56 lines per game. Frenzy mode is a huge help here.
For free powerups, I'm not sure if Lasers count as three line clears or not. Magnet is a good choice, as it pulls everything to the left, leaving it open for a straight block and helps with Tetrises for Tetris Ace. Quake is mostly solid, but the blocks that shake free aren't guaranteed to clear lines.
For paid or unlocked powerups, Avalanche fills in areas at the top with a row of three single-block columns, meaning that they always drop into holes when available. Avalanche is almost made for this; going for line clears, it becomes much easier to get 100 per game. For best results, trigger Avalanche during Frenzy.
Crusher drops a 5x5 block that sinks straight to the bottom. Note that Crusher blocks won't break up during Frenzy, so it can be a double-edged sword during large clears.