Mardu Strike Leader
 
I rather like the Dash ability (and not just because it's a reference to Rocket Powered Turbo Slug), but it does require some discretion in when and how you use it. Mardu Strike Leader, for example, has three power for three mana, which is often worth playing as soon as you can afford it. But, regardless of whether you go early or late with this guy, it's very difficult to deal with for anybody trying to rely on answer cards and one-for-one removal spells. It seems like it might be easier than it looks, but limited formats in particular are full of cards like Bathe in Dragonfire or Pyrotechnics which can be absolutely useless once this card becomes active.
 
Constructed: 3/5
Casual: 3/5
Limited: 3/5
Multiplayer: 3/5