In my opinion, I believe that the best powerup is the coin magnet. First of all, I have horrible accelerometer control, so it helps to gather and auto-collect coins. Secondly, when upgraded (mine is upgraded fully), the coin magnet can multiply coin collection, vastly expanding bonus, coins, and points. That means that triplet coins can be improved to 6-coins. I would only have to collect 15 coins to head over 100.
When mixed with other power-ups, Coin Magnet becomes a beast. When compatible with invinciblity, the player does not have to worry about obstacles. When mixed with speed-boost, the power-up becomes the most powerful power-up possible, speeding to 750m while collecting every single coin in sight.