Chavez and Taylor fought in '90. Chavez was #1 then, Whitaker was #2, and although I'm not positive, I believe Holyfield was also above Taylor (who was top 5 I believe).
http://www.boxrec.com/media/index.php/The_Ring_Magazine%27s_Annual_Ratings:_Pound_For_Pound--1980s Obviously Tyson fell off from #1, but I doubt the rest changed significantly.
As usual this decended so quickly into the abyss. A simple question with complicated and varied answers. How about amending the question to say when was the last time two boxers within the top 4 P4P fought each other?