I've been watching boxing since about 1999/2000. A lot has changed since then, but some fighters are still around. Best I can remember these were the champions back then:

Lightweight: Stevie Johnson/Jose Luis Castillo ?
Welterweight: Oscar De La Hoya/Tito Trinidad
Middleweight: Bernard Hopkins/William Joppy
Light Heavyweight: Roy Jones Jr.
Heavyweight: Evander Holyfield/ Lennox Lewis

For the sake of memory, as I was only 12 back then, I stuck with just the major weight classes of boxing.