Itachi since he was pretty much THE genjutsu artist. Kurenai was good, but he dunked on her. Sasuke may have had Tsukuyomi, but wasn't as good. Tayuya died early. And Gengetsu (the 2nd mizukage) was an edo tensei zombie and was a once and done thing. Itachi remained relevant in his craft for the series' entirety and was a looming presence in the series' infancy. He set the standard for what a person COULD do with genjutsu before Tayuya & Gengetsu showed up. (Still love them both and believe Gengetsu was stronger at genjutsu, but less influential)
2nd would have to b le Sasori for also being a master of his craft.
If you can't tell... I love control-based powers. *-* mainly illusions.