Obviously it's Zasalamel.
Why...because this is the one character in the game that demands innovation. Zass has tools for mixups that's his offence. His dmg sucks but he has a lot of mixup potential, and despite knowing this potential...it is the difficulty of actually applying it effectively in a match, that makes him very technical. Zass actually requires a lot more skill to play than most characters out there because his mixups are dependant on the player's intelligence. You don't have built-in options, like strings that end up into a low, but can be switched to end up in a mid. That mind game is something you create from scratch, the ideas are all there, but you have to actually see them, and know when to apply them.
For example in SC3 Zass 66BB hit mid into a low, as a mixup he had 66BK, a mid into a mid. He doesn't have this option any more, now you have to create that option for your self. 66BB now has to be mixuped with something like 66B 2K or 66B into 66B. This is something you have to create yourself in a match.
Most of the characters mentioned in this thread aren't difficult in technicality they're difficult in execution. Once you got the execution down...they don't need to worry too much about mixups because they can punish effectively, are safe, and or have high dmg potential. Zass works for all that, and works hard. For the most part if you can't mixup well with Zass, you will be just poking around for dmg.
If technical means technique, then Zass gets my vote. Talk about JFs, he gets most of his JF options from CH...seriously WTF. Not even rock works this hard to get dmg.
If you don't believe me, pick him up, test him...he isn't that hard to learn.