My pick would be Ray Leonard against Duran in their epic Montreal 1980 fight. I thought Leonard fought an incredible fight even though he did fall into Duran's game plan--he brawled with Roberto for the entire 15 rounds and went to places no other fighter would have EVER gone against him!! It took the performance of a lifetime for Duran to win that fight--and it was VERY CLOSE!!
Not to nitpick, but Duran was the challenger (just going with how the OP was stated) For really outstanding challenges I might say: Ray Mancini against Alexis Arguello Billy Conn against Joe Louis in their first fight Carl Williams against Larry Holmes
Patrick Ford vs Salvador Sanchez Jim McDonnell vs Azumah Nelson Mike Weaver vs Larry Holmes Yaqui Lopez in pretty much all of his title shots.
Yep--you're exactly right--forgot that Leonard had the title coming into that fight--it just SEEMED like he was the challenger LOL!! (I still think it's the greatest fight I've ever seen since I started watching boxing as a kid in the very early 1970's.)
Jersey Joe Walcott vs Joe Louis 1 Ezzard Charles vs Rocky Marciano 1 Rocky Lockridge vs Eusebio Pedroza 1 Jeff Fenech vs Azumah Nelson 1 Winky Wright vs Fernando Vargas Juan Francisco Estrada vs Roman Gonzalez 1
Billy Conn vs Joe Louis I Jersey Joe Walcott vs Joe Louis I Tim Whiterspoon vs Larry Holmes Carl Williams vs Larry Holmes Larry Holmes vs Michael Spinks II Andrew Golota vs Chris Byrd Wladimir Klitschko vs Anthony Joshua Tyson Fury vs Deontay Wilder I
Richie Kates against Victor Galindez in their first fight Tyrone Everett against Alfredo Escalera (robbery) Mickey Walker against Harry Greb (so I've read) Ernesto Marcel against Kuniaki Shibata John Conteh against Matthew Saad Muhammad in their first fight
Duran vs Hagler Kid Gavilan vs Sugar Ray Robinson 2 Arguello vs Pryor 1 Langford vs B. Joe Walcott Antonio Tarver vs Roy Jones 1 Benny Leonard vs Freddie Welsh 1 Maidana vs Mayweather 1 Conn vs Louis 1 Briscoe vs Monzon 1 Sugar Ray Robinson vs Joey Maxim Henry Armstong vs Ceferino Garcia at 160 Meldrick Taylor vs Chavez Any of the first 3 Pacquiao vs Marquez fights DeJesus vs Duran 2 Chuvalo vs Patterson Buchanan vs Velasquez (vacant European title but still counts I think) and maybe Buchanan vs Ishimatsu Junk Koo Chang vs Hilario Zapata 1 and Chang vs Sot Chitalada 2 Lomachenko vs Orlando Salido and Loma vs Haney