HBK/Taker from 1997-early 98. Very good and Great matches (Ground Zero, HIAC greatest match ever, Royal Rumble '98). The moment that started the storyline (HBK hitting Taker in the head at SummerSlam '97) was an awesome moment itself that gave off a huge buzz in the arena (I was there). That heel turn by HBK got the wheels in motion for the original DX to be formed and in the big picture of things got the Attitude era started.
Watching HBK play the arrogant, cocky heel who (who was the smaller guy and a heel going against a babyface big man, which was new ground for Vince)would egg Taker on in his promos and then have DX have his back when Taker went after HBK was the ultimate cat-mouse feud. HBK was setting the world on fire with matches and promos. Taker was finally "humanized" alittle bit and finally got the ok from Vince to sell and work like a normal person and not a supernatural deadman who barely felt anything added a whole new dimension to the Undertaker character, as well. This feud also saw the debut of Kane in the HIAC match.
Probably the best chemistry between two workers ever (if not, certainly up there). Both guys were God-like. Everything clicked from start to finish.