This is, in reference to, your general AP carries who lane solo mid. This is NOT an overall tier list. This is my opinion concerning AP carries, considering it is a role I main and considering the fact that I know how to play most of these champions. This tier list is focused on solo queue and competitive play, based on what I have seen and what I have expirienced at 1700+ elo. Organized left to right from best to worst, changes marked in bold. If you want to comment on the list, do so in this thread or on my blog.

Tier 1: Karthus, Xerath, Cassiopeia, Kennen, Morgana, Annie
Tier 2: Gragas, Zilean, Orianna, Sion, Kassadin, Ryze, Fizz
Tier 3a: Brand, Heimerdinger, Vladimir, Soraka, Karma
Tier 3b: Nidalee, Twisted Fate, Katarina, Leblanc
Tier 4: Malzahar, Veigar, Lux

Tier 1 - These champions are elite AP carries due to the fact that they are solid in all phases of the game and they each offer their team unique utility.
Tier 2 - These champions are also strong in all phases of the game, however their weaknesses are easier to exploit than that of the tier 1 carries.
Tier 3a - These champions are niche champions. They have a single general strength but are not the full package. Having said that, when in the hands of a good player these champions can carry games and preform at the level of a tier 1 carry. You will generally see these champions succeed more in solo queue than in competitve play.
Tier 3b - These are also niche champions who can be very strong picks for a team. However, these champions generally require a specific team composition for them to shine, as well as a specific strategy. These champions are seen a lot more in competitive play, and can be as strong as tier 1 APs when utilized correctly.
Tier 4 - Unless you are nyjacky, you will find these champions to be outshined by champions in higher tiers. These champions generally have a strength and can still carry games, however other champions just do it better.

Change Explanations

Boy, this list is starting to look really top-heavy huh? That's because riot has been doing a great job balancing the game recently and I feel that a vast majority of champions in this game are viable. +rep to riot.

Kennen moved UP to tier 1 - While compared to casters in tier 1, kennen is the most beatable in lane, I just can't hold him out of this tier any longer. Take fid's ult, multiply it by 2, and give it a stun, and you have the beast that is Kennen. His late game might be unmatched by any AP carry right now. He is capable of beating a lot of champs in lane early game, but I still think that he is very beatable in lane.

Gragas moved DOWN to tier 2 - Gragas can still burst with the best of 'em, but I feel this is a necessary change. His damage output is unreliable and for that reason he's not as rediculous as some other APs. He also suffers from the same melee syndrome Kassadin does levels 1-6 which makes his lane phase difficult. Overall too unreliable to stay in tier 1.

LeBlanc moved UP to tier 3b - Leblanc looks like she can be a counterpick in competitve play based on MLG. Reginald played her twice at MLG (that I saw). The first game, he didn't get many kills and he was an afterthought late game. His combo did little to no damage even to squishies. The second game TSM used a more split push/poke oriented playstyle and regi's Leblanc worked out a lot better that game. The leblancs I have seen in solo queue haven't really impressed me much. I think she can be good, but she needs a good comp/playstyle to be successful. Passive play early game can shut her down.

Malzahar moved DOWN to tier 4 - I've started 10/0 as malz only to lose the game. More than once. It's really sad what has happened to malz, but he's just countered too easily. He puts down pool? It's easy to step out of it. He ults you? CC him/qss. You gank him? He's dead because he has 0 escape abilities. His lane phase is super strong but his teamfighting is pathetic.

Orianna moved up in tier 2 - Orianna's buffs really helped her and I feel she is entirely viable now. Still one of the best ults in the game and really, really strong poke. When played smartly her laning phase should be no problem. Orianna has an insanely good kit for an AP carry and really solid damage. She's a utility caster who is on par with Zilean.

Sion moved down in tier 2 - Never thought Sion was OP in the first place, and a scaling nerf to an already fragile late game really hurts him. He's beatable in lane and quite honestly tier 2 may be generous placement for him. His early/mid game is still very strong though.

Fizz added to tier 2 - I've seen a few fizz players, he seems like a decent champion with some potential but I'm not quite sure if he is on the level of tier 1/2 APs. I'll throw him in tier 2 for now until I get a better feel for him.

Soraka and Karma added to tier 3a - Both these champions are capable of soloing mid and carrying games, however both champions really lack damage output. In a bursty meta two squishy carries with no burst can really end up hurting your team. Having said this midraka and karma can both win mid with relative ease and offer unique utility that would be foolish to ignore.