Another one that deserves a shout is Tony Zale.. the war sidelined Zale for 4 years yet he came back & gave us one of the greatest trilogies of all time against Rocky graziano.. Vitalis Klitschkos late career comeback deserves a mention too..
Willie Pep coming back from almost dying in a plane crash. He had to wear a full body cast and against the advice of the doctors he was back in the ring five months after the crash to continue his amazing career.
Vitali Klitschko had a far better old guy career and comeback than Foreman. Hopkins also got written off plenty of times, only to win and defend more titles.
It really that Foreman did it after 10 years off. Vitali's was impressive too though considering all his injuries and not having a active fight in 4 years.
Foreman's success was contrived under special circumstances. He didn't beat any contenders but was granted three title shots* in a short period. The first was against an undersized champion who was easy to hit. The second was against a chinny guy with a crappy gas tank. And the third, when the stars aligned, was against an undersized champion with a crappy chin and sedentary footwork. Foreman won because of privilege and failed to do anything with the title. Vitali stepped straight into his title shot via WBC rules and went 10-0 in title fights before retiring. *George and his Foremaniacs would've counted the WBO as if it were a golden Ring title, so it counts as a title shot for the purposes of this discussion.
Foreman was out of it for 10 years! Nobody has done anything like it before or since and nobody ever will. Vitati was out for less than 4 years. I think even Vitali himself would laugh off any comparison.
Some great ones mentioned already. I think Jeff Fenech coming back off a 2 year retirement due to hand injuries and moving up in weight to whoop Azumah Nelson in Las Vegas deserves and honorable mention.