At the time, I thought Kane was disposable Undertaker foe #114 along the lines of The Executioner. Definitely was proven wrong on that one. Also Vince's commentary took away from it for me.
I suppose there's also the factor of a guy having an established track record versus being a complete unknown.
He was supposed to be a one-off for Taker to go over iirc, but Glenn Jacobs was so good in the role that he got 20 years out of the Kane character. Amazing, really.
I'd put the Shield and Nexus debuts in the top 5 easily.
I'd put The Shield in there. Nexus just leaves a sour taste in my mouth because of how that turned out.
Jericho is the best debut of all time imo, though.
Shinsuke for the initial reaction is definitely up there, and I doubt WWE is going to **** up his booking -- mostly because he's on Smackdown. Maybe he's top-5 debut, it's still too early to say, but it's going to take a lot to top Jericho's imo.
On another note, Shinsuke's debut was at the perfect time. It was at 9pm for a segment that advertised calling out Cena. Imagine sitting at home and seeing that big time entrance and reaction to this huge star who oozes charisma? Instantly hooked.