Vel'koz, IMO, who is the best immobile mage in the game.
You can hit from weird angles - and if you do manage to land your full combo, you will devastate people.
Even if you miss your ult initially, you can always swivel to do SOME damage, maybe even get a full combo of his passive off - if you whiff on Xerath, which is easy to do, well, that's just another cooldown wasted.
Vel'Koz also isn't super item dependent. He has high base damages, and that scaling true damage passive.
Immobile mages are very reliant on your team to not be doing crappy, but even then you can carry with Vel'Koz - you can turn entire fights around with a well placed knockup+w+ult combo, saving q for fleeing stragglers.
Then again, I am extremely biased at this point. I've played several hundred games of Vel'Koz the past few months.
Also, to the below poster, Vel'Koz is not reliant on Rylais, so if it gets removed this coming pre-season, he is not harmed in the least.