Nathan and Kelly were really awesome when they were a kinda item :P

I picked series 1, just because the whole Simon/Alisha thing annoyed me a little. Though both series are equally good in terms of storylines. Misfits is one of these few rare shows that keeps on getting better each season.
I'm eager to see how series 3 will be like, if the writers will be able to keep our interest although Nathan is no longer in the show.

S2 was good too, but I have to go with what Desaday said.