Chun is the hardest character for me in the game. Guile's loops are difficult but, at least as far as I know they are all he has that's hard. Akuma is pretty difficult when you start bringing Demons into the mix. Of course, difficulty in SFV is relative. In terms of static execution (training mode) I haven't really found anything I cant do with some basic practice, I just dont have the patience to get it down enough to use in matches, or commit *all* of the variations to memory - which is the case with Urien. I can see and repeat his sequences easily enough in training mode.
In USFIV I struggled with execution. It was my first FG and I had no idea wtf I was doing. the first 6 months or so I played that game I was held back immensely by my execution, even though I was practicing my ass off.. Then I picked up KOFXIII on Steam kind of randomly, after hearing it had crazy-hard execution, as a means of general practice.. KOF kind of redefined what I considered hard and I was able to improve a lot by practicing some of the batshit KOF stuff.
Corner Saiki combo as an example (obviously they arent all this silly, but quite a few are)
st.C xx qcb+D (hold D), j.4B, dp+B [DC] qcf+C, qcb+D (hold D), j.4B, qcf+A, dp+BD/qcfqcf+K