That's a tough one... I'll go with triple reds. You can use them in front and behind you, you can still shield yourself from shells coming at you, and you can keep them around for a while and wait until the right time to use them.
POW can make everyone in front of you lose their items, however it only works on those in front of you, and you are defenseless from the racers trailing behind you.
By the way, this should've been a poll in my opinion.