I wouldn't say "master" is very realistic. I've been at this for awhile & I'm still finding new stuff/juggles/patterns.
Ask this anywhere & you're going to get 200 Lei's... based mostly on the idea that he's got about 10 stances, effectively... more if you count each variation of being on the ground. You've really got to know where each move with Lei can take you... I know "enough" to at least be dangerous with him on random or Mokujin style, but I wouldn't say I'm close to master.
Hwoarang is another one. He doesn't look too complicated at first but in order to not be predictable, you got to hammer out lots of additional technique with all of his cancels & crap. I've never been gwhat I would call reat with Hwoarang. Just seems like too much effort for a mediocre character. I like Baek more but he may get phased out come T7.
When you "master" a character, you can consistently do all of they're stuff. This leads down to a few hard characters. Hwo (JFSR) Bryan (Taunt JU) Kaz (PEWGF). However none of these characters gets to the level of doing a non-buffered iSW. This alone makes King, Armor King and maybe Lars the hardest characters to master. Now then, Armor King has DU. This can be done in 14 frames, and from that, we can easily say that, Armor King is the hardest character in the game to master.
On another note, back-turned FLEA from Yoshimitsu, is the single hardest move to do in the game. Go try it, ff+1+2, b+1+2. It's a justframe, and the timing is insane.
Official Advanced Lawnmower Stay away from Lux. She's useless.
I would interject that just because a move is complex doesn't mean it is practical. There's a good number of strange secret moves in here, but stuff you would only mess around with when playing with friends.
I'd throw Miguel on here too... I have a few good tricks to work with but I don't have a great idea how to play the guy. He seems about as effect as a default setting Combot. Its more of an aesthetic thing, but Miguel always looks like he's exerting way too much energy, swinging about the way he does.
Zafina gets allot of flak for being easy to knock out out of her moves & inability to block in her stances. I suppose her confusing movement helps a little, but if someone is familiar with her moveset, you're not fooling anyone. I want to say there's a good character in there but I'm skeptical.