I like playing with assassins.
Katarina and Vayne are among my favorites.
Katarinas burst and Shunpo goodness for escape and chase are sooper dooper.
For Vayne I love how much she is rewarded for not quiting an attack, with silver rings and the bonus movement speed and whatnot.
But alot of the time Im just using the free champions, so that I can better know their abilities, and how to counter them.