Itachi's genjutsu J-man's takes too long as Ma and Pa have too set up their vocals Itachi can place you in Genjutsu with his finger and even use your senses against you too trap you in a genjutsu.
Ma and Pa genjutsu (NOT J-man's) takes a bit prep time but can be a 1-hit KO technique while Itachi's is immediate but not so effective and requires eye contact. For me its Jiraya's because of its effectiveness.
Jiraiya has the most deadly genjutsu in the entire series until proven otherwise. But yes, Itachi can cast his genjutsu's a lot easier than Jiraiya can cast his.