Is this a trick question?
Ali is greatest boxer ever, regardless of any era.
Now, Sugar Ray Robinson, Henry Armstrong & Joe Louis are all also in the discussion, but there's no question Ali's skills transcend all era's.
His speed, Defensive style, footwork & raw punching ability was spectacular. His reach was one of the best tools a Heavyweight ever had & his ability to break his opponents down mentally is what gave him an unmatched edge.
He also has arguably one the best resumes ever in Boxing history, he Sonny Liston twice, Floyd Patterson twice, Karl Mildenberger, Jerry Quarry, Bob Foster, Ken Norton, Joe Frazier, George Foreman, Ron Lyle, Earnie Shavers & Leon Spinks. And had he not gotten sick he would've whooped Larry Holmes & Trevor Berbick.
As far as Heavyweights is concerned there is none greater than Ali & he fought in the greatest era of Heavyweights ever. Had he fought in his prime in today's era which is legitimately one of/if not the worst era of Heavyweights he would breeze through the division without any trouble whatsoever.
There's not a single Heavyweight in today's era over the last 15-20 years who could even come close to being halfway as good as Ali was. He would demoralize every single one of them today & it's not even close.
