A Few Words...
This tier list is my vision on the current mid lane play. It's not entirely based on my own experiences but also on experiences of other people and things I've seen in tournaments and high elo streams. This is not a specific soloQ or TeamQ list. This tier list is based on the potential of each and every champ if (s)he is played by a player who knows what he is doing.
The best of the best. These guys have zero to no direct counters in mid lane and perform extremely well in all stages of the game. They have the ability to carry games by themselves and have high kill potential.
Very viable champions overall. They have everything to be amazing carries yet aren't perfect compared to the tier I champs. They either aren't great in all phases, lack kill potential or lack range.
These champions are still very good but overall excel at simply one thing or they are simply outclassed by others who are in higher tiers.
These champions perform well in specific match-ups but are overall heavily outclassed by others. They do not perform amazing in all stages of the game but rather in one part of it. Most lack CC which is also a huge setback on an AP carry.
Wouldn't necessarily call these champions bad but it's simply outdated and outclassed. Don't get me wrong, one can still perform well with these champs but they in no way excel at anything and are incredibly item and team dependent.
They can do it, don't!
Placement of Specific Champs
Karthus - Tier II
While not having an amazing laning phase, he's still pretty high up because his late-game is so ridiculously strong. He's placed in Tier II because he lacks killing potential in his own lane.
Cho'Gath - Tier II
While Cho'Gath isn't often played as an AP mid, he's definitely capable of it and he does his job well. He is very tanky and has amazing burst thoughout the entire game. His only thing keeping him from being Tier I is that he gets countered if the enemy team has a lot of mobility.
Malzahar - Tier II
It seems very high rated for such an underplayed champion but I think Malzahar is still one of the better mages out there. He pushes hard, has a lot of damage and is very item independent. Also, he allows for fast Baron Nashor / Dragon pickups and has a very powerful lockdown utlimate. True reason for him being Tier II: Roaming! A roaming Malzahar is incredibly powerful and allows for huge pressure on the lanes.
Cassiopeia - Tier III
"Cassio Tier III? This must be a mistake!" Unfortunately it isn't. In my opinion Cassio is still very good but definitely lacks kill potential and get's outclassed by champions like Orianna and Zyra. Her laning phase is very strong but her teamfighting requires very precise potioning and timing for her to be effective enough.
Viktor - Tier IV
Don't get me wrong, I love Viktor! I love his laning power and his teamfighting but we can all conclude that it isn't enough. The main drawback on Viktor is that he lacks range if maxing Q and lacks early-game presence and has mana problems if he maxes E. The main reason he is this low, is because the cool down on his ultimate really cuts him. It's not up when others have their ultimates, setting him back.
Vladimir - Tier IV
He has a load of counters (e.g. Malzahar, Cassiopeia, Viktor) and has absolutely no CC. He doesn't have the burst other AP's have and the lane is too short to get off his DPS effectively. His late-game prevents him from dropping down more.
LeBlanc - Tier IV
"Wait, is LeBlanc seriously Tier IV with that ZOMGBurst?" Yes, the reason is that she is really easy to counter. With the right runes, actually most mages have the capability of beating her in lane. I'm not saying she is really bad but she is just really easily countered and she doesn't really have that much of a late-game.
Kog'Maw - Tier V
Kog'Maw as an AP mid seriously dropped dead in my opinion. He has been nerved several times on his main damage dealer which has destroyed him. His laning phase is rather weak, he has no hard CC and has to build squishy early despite having no escape moves. He fits well in poke compositions but meh, that's it.