Thresh and Varus to me don't have a very high learning curve whereas Lucian and Zac take a bit more to get at least decent with and finally Kha and Zed are the toughest to get a good grip on but seem to be very rewarding. That said, I don't think Zed is all he's cracked up to be (albeit I'm still trying to up my proficiency with him). But objectively he feels like he blasts one person out of a teamfight then has to hang around the fringe and mop up. I find that (again, objectively) Kha can be far more effective in teamfights.
Mind you I think both are not your best pick in teamfight lineups compared to ganky lineups but Kha feels like he'll bring more to the table with a more reliable slow and better escapeS (capital 's' on purpose).
In terms of fun I think it depends what you like.