My favorite Jericho remains WCW heel Jericho.
That said, the most over Jericho was absolutely 2000 Jericho.
If they had pulled the trigger on him in 2000, not necessarily in that overturned TV title match, but later on, maybe had him with LMS against HHH at Fully Loaded and put him into the title picture that summer... they would have had a MASSIVE draw.
I think they waited too long to give him the title as a heel in 2001, and then his title run was trash.
Wasted opportunity.
2008 Jericho was alright, but I feel like pretty much anybody could have done that character. It was intriguing from Jericho, since he basically did the opposite of everything he was before, but there was nothing inherently interesting about the character beyond that.