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.

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

Xerath moved to tier 1 - Been playing a lot of xerath lately and yeah, he's ******** op. Burst damage of annie with the highest range capabilities in the game. You don't even need to use your W to be a strong caster, but it allows you to have really strong pokes and farm creep waves from a safe distance. His W also has a 4 second cooldown when maxed. This guy is just rediculous.

Sion moved to top of tier 2 - Some people feel sion is OP. Personally I don't think he is at the power level of other AP carries but sion should certainly change the way you play against him. His burst is not 100% to 0% late game (especially if you have MR), which it needs to be because Sion basically is forced to suicide to kill a squishy target. He can break games early/mid game but because he is not strong in all phases I just can't justify tier 1 for him.

Orianna added to tier 2 - derped and didnt add her in the first addition. Annyways Orianna is a really strong pick, for some reason she is very underplayed though. I feel this is mainly because she requires a lot of skill to play properly, but when played correctly she is 100% viable and her lategame utility is rivalled by only zilean. Gamebreaking ult, great initiating abilities with EW, just an overall strong pick. Her one setback is her range gimping her early game but it really isn't that bad if you utilize her ball correctly.

Let me know in the comments if you have any questions.