Originally Posted by Vengeance
In terms of actual skill they would be considered equal however Itachi has natural resistance & ability for genjutsu because of his sharingan. I'll try to dumb this down for the retarded. Two boxers with equal skill get into a fight. One boxer is 56 years old 5'4" 140LBS while the other boxer is 27 years old 6'3" 205LBS. Guess which boxer is going to win?
Itachi even without sharingan is faster at casting genjutsu than Kurenai, his finger genjutsu didn't require hand seals and he casted a two layer genjutsu on Naruto while fighting Kakashi at the same time. Itachi's hand seal speed is faster anyway so even for genjutsu that requires hand seals Itachi would be the one to cast the genjutsu faster if we assume Kurenai and Itachi have the same genjutsu knowledge.
With sharingan, it gives the extra advantage to Itachi because he can cast more genjutsus without hand seals and it gives him a defense against genjutsu.
Lastly, Itachi is probably smarter than Kurenai and that gives him an advantage in a genjutsu battle because it affects the way the genjutsu is used.
So, Itachi without sharingan vs Kurenai in a purely genjutsu battle, winner is Itachi.