Main's primarily Kennen for both solo top and mid. I love how cute he waddles around and that stun and ult.
Others that I play a lot are Riven, Cait, Rammus, and Teemo.
Riven because she's such fun.
Cait because dat snipe.
Rammus because TANK EVERYTHING.
Teemo because he's freaking adorable and them AP shrooms.