They hit the big time/money fights, after facing legit challengers, still faced the very best at their best and never shied away from a legit challenge?
I'm guessing lots of great fighters, perhaps the majority of them, "never shied away" from a tough challenge, but that's what managers are for.
You gotta be joking.. Lennox Lewis avoided every heavyweight contender throughout the 1990s who was "At the top of their Game" Bowe 93 & 95, Foreman 94-97, Witherspoon 95, Tyson 96, Hide 95, Moorer 97, Holmes 92/3, Sanders 00, Ruiz 98-01, Brewster 99-02, Byrd 02, Wlad 0-03, Vitali rematch and many other top contenders. Lewis chose to fight, Mavrovic, Akinwande, Jackson, Morrison (hiv), Botha, Briggs, Rahman, McCall, Fortune, Butler, Bruno between 1992 - 99 all lower class fighters who were all exposed and over the hill.
Although not always champions but never ones to sit and idly wait to be given a shot at a title, Saoul Mamby and Juan Laporte never shirked a challenge while they were occupying top contender status. Their non-title fight roster would embarrass many a champion.