Personally, I am not a huge HBK fan. Don't mistake that for not admiring Shawn Michaels' carrer as a whole because everything he did in the ring was second to none. He feuded with the whos who of professional wrestling (HHH,Austin,Bret,Taker etc.)
It's so hard to pick a greatest moment or match, he had so many. The match that I hold in the higest regard is the Casket Match @ The 1998 Royal Rumble. I feel that this match is almost overlooked by the Wrestlemania and HIAC matches between the two. This match is definetely one of my top 10 matches all time. Many people say that Bret/HBK was his best feud. I would say it was the most personal, but the short feud between The Undertaker/HBK was fucking brutal.
I would have to say that the match with Trips @ SS 2002 may have been the biggest of his carrer though. Many people though that he was done, had nothing left. In that match he proved that he still had it. That was at the time that I was a huge Triple H mark and as that match progessed I remember thinking that HBK wasn't going to be the same HBK of the 90's.... He proved me wrong.
For me though, the biggest match he had in terms of industry impact was at Wrestlemania XIV. It was the changing of the guard so to speak when Austin beat HBK to become WWF champion. HBK was without a doubt the top guy in the company at the time. Austin who was still for the most part a mid card guy really proved his staying power in that match imo.