What do you think are the best, bursty, snowball champions? Since I personally like Akali & Diana (But don't play Diana much anymore after the massive nerfs) I need to broaden my horizens, since I like having a lot of burst AND being able to carry a not-so-good team!
My vote's for Riven. High mobility tends to lead to strong snowballing potential, and Riven excels here because her effective engage range is higher than just about every other champ in the game. That is to say, she can begin engaging on you from fairly far away, and even though you'll start running away, it's almost inevitable she'll reach you.
Most mids aren't nearly as bad, because they're fairly dependent on blue buff to spam their abilities, and someone like Morgana (and to an extent Orianna) can safely farm without ever entering engage range. This is partly why top lane is so snowbally... it's very easy to zone someone out due to the long distance between turrets.