I'm surprised no-one has already posted a topic on this, but what are peoples favourite weapons on multiplayer? The most erratic is the banana. Sometime it leaves you with several columns requiring I pieces, but sometimes it helps, changing your screen to give you a single column by the side
I would have to say Lightning because if the opponent chooses to just stall and not lay any pieces, you've gained a ton of time on him and should be able to send him an extra tetris. Plus, if you strategise, you should save it for right before you make a tetris, so that you can effectively send two tetrisses by the time he gets out of it. If he Desides to actually try to place blocks, he'll probably end up creating covered holes with his pieces.
The worst combination to get hit by is the mushroom and lightning at once - it can seriously screw you up, especially if you're more than halfway up the screen