I would really like to see what you guys say. In my opinion it has to be Teemo aka The Spawn of Satan, not saying he is really OP or Balanced but anyone that picks him up, even for the very first time can do incredibly well.
Depends on what you mean by "consistent" because a lot of that consistency relies on the player's ability as well. Do they know what to do with this champion, are they going to make stupid mistakes that would be dumb on any champ, etc.
That said, assuming stupid mistakes and the like are kept down to a minimum and we're not looking at support picks, I would say Caitlyn. She outranges everybody in bot lane at level one and that's just HUGE. She has a kit that can easily harass and push lane(Q), synergize with support aggression (range+trap), keep her safe from bad engages or ganks (net) and secure kills from afar (ult). She's a little slow, but if you don't play ADC much, I would always recommend learning Cait because she's good in practically any matchup.
is a pretty consistent champion IMO. I can play her top after weeks of not using her and still win lane. Her ADC is kinda eh but it can work for a lot of match-ups. I think her true strength comes from the burst of her Passive proc>AA>E>AA, so if you're planning to pick up a new consistent champion, she's someone to look at.
lose lane win game
Most of the ones posted already are quite consistent. The only one I would add would be 
. He's always been remarkably consistent, stun people, shatter, wack, run away.
I almost always have good results with nunu. Short of an afk or ultra feeder, I usually win when I play support or jungle nunu.
is the anthesis of what you're looking for IMO. Either he's an insane assassin who kills you, steals a buff and is out before you even realize he's there...or he's a literal asshat.
Hey Folks,
I'd have to say that Skarner before the rework was such a consistent champion. If you looked at his potential in Top lane with a Tri-Force ;) I honestly believe he had to potential to do anything. For example, Great damage outputs, Great Tanking ability, Decent speed, Secures in teamfights and not to mention that extremely valuable Ult.. Just plainly a great champion as far as Mid and Late game was concerned. Yes his early game was weak due to problems with his Mana but that was compensated for an amazing rest of the game. Every thing about that champion was great! That is of coarse, Before the Riot nation attacked.
Cheers guys,
Dave
For ADC, I'd say 
for the same reason Abiwon did, and I'd probably add 
He's super safe, got a well-rounded if not particularly fancy kit, and does ok in pretty much every situation, plus the map control of the global ult. His main downside is that he doesn't have any natural cc. 
is in a somewhat similar place right now. For mages, 
If you're looking for someone who can consistently do everything, ever, 
I really have to say wukong. From his escape(w) or initiate(R) or lane combo(E,Q) he can go tank duelist or assasin. Easy to play very fun and a good team fighter or just someone to go ham with
