In my experience the hardest champions to kill.
Kassadin, Ahri, Nidalee, Lux, Skarner, Shyvana, Jarvan, Yorick, Anivia, Kennen, Caitlyn, Teemo, Kayle, Akali. In some order. At least those are the ones I hate ganking. They can pretty much escape most everything.
Edit:I totally Forgot Janna, Trist and Corki. Corki and Janna more so then Trist.
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Whatever...