Random random random, only character I haven't played a full game with yet is Clockwerk, for some reason.
I've got plenty of favorites, but I'd have to say Slark is definitely high up there on the list. Also Ogre Magi, Treant, Rubick (of course), Lifestealer, Doom, Jakiro, Pugna... So, yeah, a few. Slark is the only one so far I've actually starred in-game though, so I guess he's my main man (that or I'm just lazy).
Also, how on earth are you supposed to do Spectre? I've never been able to do much of anything useful the few times I've played her. I can understand how Dispersion is an awesome passive, but Desolate just seems so tricky to pull off. And her ultimate is equal parts awesome and useless.
If you think Ion Shell is hilarious against melee players, try it against the bots. Bots flip out completely and just start running away, regardless of how much health they have. And they also run directly away from you, even if that means charging into your tower. They also refuse to use any skills until they're out of the ion zap range, including hard disables or TP's that would save their lives.
I once got 14 kills in the first 12 minutes as DS. I think the bots are a TEEEEEENSY bit smarter these days though.