#2 Big Show (before)
After an impressive stint in WCW, Big Show made an explosive debut at WWE St. Valentine's Day Massacre: In Your House. He interfered in the Steel Cage match between Stone Cold and Vince McMahon, throwing Austin on one side of the cage, This led to the cage breaking and Austin falling to the floor, thus winning the match. Big Show cemented himself as a heel and was a major WWE attraction for the next several years.
Big Show is a 4-time World Champion in WWE and has had memorable rivalries with the likes of Brock Lesnar, John Cena, and The Undertaker. For the past four years or so, he has been making part-time appearances for the company.
#2 Hulk Hogan (after)
On the road to WrestleMania 18, Vince McMahon promised that he would kill his own creation, WWE, by injecting it with a dose of lethal poison. This turned out to be nWo, WCW's notorious faction. Hulk Hogan, Scott Hall, and Kevin Nash made their WWE debut as nWo and Hogan went on to have a classic with The Rock at WrestleMania 18.
Hogan's popularity was still soaring high, which prompted WWE to give him another WWE title run. He won the title by defeating Triple H at Backlash 2002, and lost it soon after to The Undertaker, at Judgment Day.