Most of these have obvious answers, but
Eddie
Benoit
Sid
are good questions. I like Sid's mid 90's WWF run, Eddie's WWE run, and Benoit in WCW.
Edit:
More good ones mentioned
Hogan- WWF
Nash/Hall- WCW, Nash's run as Wolfpac babyface was his career highlight and he had more memorable moments than as Diesel. Same with Hall.
Booker- WCW, he was a better worker then and still over without the same push/airtime. The King Booker gimmick was too goofy to me to be serious and not stupid enough to be funny, he was at his best as a pure babyface who got over with his in-ring skills (and got over big in WWE like that too, before the HHH incident)
Rude- WWF, that's hard though
Eddie
Benoit
Sid
are good questions. I like Sid's mid 90's WWF run, Eddie's WWE run, and Benoit in WCW.
Edit:
More good ones mentioned
Hogan- WWF
Nash/Hall- WCW, Nash's run as Wolfpac babyface was his career highlight and he had more memorable moments than as Diesel. Same with Hall.
Booker- WCW, he was a better worker then and still over without the same push/airtime. The King Booker gimmick was too goofy to me to be serious and not stupid enough to be funny, he was at his best as a pure babyface who got over with his in-ring skills (and got over big in WWE like that too, before the HHH incident)
Rude- WWF, that's hard though
but in WCW he had the more well known matches, the title reigns, and being brutal as fukk..dude had classics with sting, cactus jack, nikita koloff, and ric flair..he had an above average match with big bossman and pulled the last great match out of hacksaw jim duggan

His ECW run shyts all over his WWE run