I'd personally go with Christian vs Jericho at WM20 for a number of reasons.
-Had a very emotional story about a long standing tag team (they had tagged on and off together for the previous 2 years) splitting up due to a love triangle between the two of them and Trish Stratus, an angle that hadn't really been done in the 'E' for a while, and hasn't really been done since, especially not in the way they did it for Christian/Y2J, where Christian actually attacked Trish during a match that Bischoff put them in.
-Christian's abilities and talent really shone through in this fued. He wasn't in any form of gimmick match or relying on weapon spots, and a lot of people suddenly seemed to realise, 'Hey, Christian's actually pretty damn good at wrestling.'
-Personally i feel this match, and the end result, were both a lot better than Rock n Sock v Evolution, despite the fact that that fued had been building for 6 months straight (a full year if you include the original punt down the stairs). If i had to rate every match on the WM20 card in terms of which ones i enjoyed and which i didn't, i think Christian v Y2J was only surpassed by the World title matches, and that's a big thing for me to say considering that Taker returned that night, and obviously i'm an uber Taker mark.
And finally, i'd say this is one of the most underrated matches of the last 10 years, because i seem to be the only person who enjoyed that match. Don't think i've ever read anyone else state that they appreciated this particualr match in the same capacity that i did.