#1 Chris Jericho (before)
Former WCW Cruiserweight Champion Chris Jericho's appearance on an episode of RAW in 1999 is widely regarded as one of the greatest debuts of all time. He introduced himself as a guy who had come to save WWE. Jericho had a verbal exchange with The Rock in one of the most memorable moments in RAW history.
He had a way bigger and successful career in WWE than he ever had in WCW, and won several World titles during his stint. Jericho is still going strong as a top star in AEW, more than two decades after his WWE debut.
#1 Goldberg (after)
Former WCW World Champion Goldberg made his WWE debut on the RAW after WrestleMania 19 and speared The Rock, the man he later defeated at Backlash 2003. He won the World title from Triple H months later and went on to defeat Brock Lesnar at WrestleMania 20 in his last match.
Goldberg returned to WWE in 2016 and squashed Lesnar at Survivor Series. He has won the Universal title on two occasions since and was inducted into the WWE Hall of Fame in 2018.