I reckon around the shavers fight time for sure. By then he was really slowing. A lot of punches he was getting caught with off shavers, just a few years early he'd have probably avoided. Old George would have picked away at Ali whilst taking his Best shots. Foreman by decision id say .
Interesting question .. post Norton 3 .. Depends on if that Ali trained as he rarely did in his absurd second reign .. fascinating to note how physically talented and tough Ali was that even this faded, unmotivated, vain version of himself beat Bugner, Lyle, squeezed by Shavers. (Although got a gift vs Norton) .. tough call as he dropped so much between fights but how much is hard to say as he did not remain .. I'd say for sure the rematch Spinks , where he was worse than the first fight Spinks ( it was Leon who collapsed ) .. maybe Shavers and Evangelista too ..