Off the top of my head: (no particular order)
-Triple H return from injury January 7th, 2002.
-Jericho's first WWE appearance 1999
-Undertaker's return from injury Judgment Day 2000
-The Rock's return before Wrestlemania in 2004....when Foley introduced The Rock to Orton, Batista, and Flair.
-Vince McMahon's return in 2000 when he knocked out HHH (not me), knocked out Shane in the ring, and helped The Rock defeat Big Show to qualify for Wrestlemania's main-event.
-Gene Snitsky.....
-Austin's return in 2000 after a 10 month absence.
-The Rock's return in 2003 as a heel.
-Shawn Michaels being announced as the newest member of the nWo
-Shawn Michaels being announced as the new commissioner of the WWE in 1998.