I'd put Flair and Jericho ahead of Guerrero and Savage, personally, but there's thin spaces between them.
1. Hulk Hogan
He was on top in both companies during some of their biggest periods of income and relevance, so of course he's #1. Rock n' Wrestling doesn't work the same without Hulkamania, and the nWo doesn't have anywhere near the same impact without "Hogan's a BAD GUY now" being a factor.
2. Ric Flair
He was, arguably, the top draw for JCP/WCW in both companies history. Ratings and gates may have seen greater heights with Hogan, but the WCW crowds chanted "we want Flair" whenever he wasn't around. His WWF/E runs are also quite eventful, between the '92 Rumble and his run up WM24.
3. Chris Jericho
Wildcard pick, as far as placement goes, but even the nature in which he left WCW (and all of the backstage headaches) didn't detract from his being the best non-nWo midcard heel for 1998 and 1999. Then he gets to the WWF and becomes a major player that gets shuffled around the card and almost always delivers, whether it was the stuff with the Radicals and HHH in 2000, being the first Undisputed champion, or his epic 2008 "best in the world at what I do" run. He may have petered off during those later runs, but that he was able to stay relevant and deliver memorable moments and matches in both companies over the span of 17 years is insane.
4. Randy Savage
The man who set the bar for a great WrestleMania performance, topped the card in both companies, and delivered great matches.
5. Rey Mysterio Jr.
THE focus of the Cruiserweight division, one of the hottest midcard acts in either company, and a couple runs at the top in WWE. Even shyte booking in both companies couldn't stop him from standing out.
6. Eddie Guerrero
I'll be honest, he's this high mostly because of his WWE run from his 2002 return up to his death in 2005. He was always a great in-ring talent in both companies, but his WCW work was always marred by weak promos and bad booking, not to mention his demons. When he leaned into the goofier side of "Latino Heat" in the WWF is when he really shined and put it all together, and that run from the SmackDown Six up until the "Dominic" feud is the stuff of legend.
7. Kevin Nash
I'm surprised he isn't being mentioned more, because even though his ringwork was often shyte? The dude was OVER and pushed to the moon. He was a major player once he showed up as Diesel, literally kickstarted the hottest act in WCW history (thereby forcing the WWF's hand on a few key changes), and held the top belts in both companies. His return to WWE in 2002 may have been garbage, but his 90s runs were important.
8. Booker T
Half of a top tag team in both companies? Check. World title holder in both companies? Check. Very over midcard worker in both companies? Check. He survived Russo's worst booking period and came out the other side as a genuine top talent that even diehard WWF fans admired, and I'd love to see the alternate universe version where he isn't treated as a jobber joke for the first few years of his WWE run and is handled appropriately from the get-go.
9. Mick Foley
His WCW work was met with brief bursts of excellence (the feuds with Sting, Vader, and the Nasty Boys) but marred by terrible booking (Lost in Cleveland, dumping him like he's hot trash once Hogan came in), and his WWF work is what cemented his legacy in the business. He's only this low because everything that was great that he did in WCW was ruined by the constant cycle of bad booking choices in the office.
10. Ricky Steamboat
He's this low because of how brief his WWF runs were (and the less said about that second one the better), and because his 90s WCW run ended prematurely. A handful of classic matches (WM3 with Savage, the Flair series for JCP), a great hand that aided younger talent in WCW (particularly Steve Austin and Shane Douglas), and just an all-around respectable guy in an era of the industry where most of the top talents don't fit that bill.