Re: Strongest character Hulk could KO when he faces them in a boxing match.
I presume, given your choice of comic character for this question, you're looking for matches involving power punches. Then you want to use Worldbreaker Hulk.
And the toughest opponent I can see him taking out with or without his so-called "Guardian Amp" in a boxing style match is Superboy Prime.
Now, for your presumed real life love of the sport.
Type in "Rumble in the Jungle" into YouTube and watch one of the full versions of Muhammad Ali versus George Foreman.
BEFORE you do that though, to actually appreciate what you're watching, I recommend seeing
1. George Foreman versus Ken Norton
2. George Foreman versus (Jose?) Roman
3. George Foreman versus Gerry Cooney
4. George Foreman versus Joe Frazier 1973 (1st fight)
... in approximately that order.
If you've only followed the sport from about the last 20 years or so, I think you'll be pleasantly surprised by what you see. Just watch those 4 matches above and FINISH by watching a full version of Foreman v Ali.
Most highlight reels suck. I quickly collated some decent free vids of the full fights listed above. Hopefully the links last awhile. While they do, enjoy!
I suddenly wondered, having re-viewed the following tonight, how much of the following Ali knew. Might what MJW teaches here have been one of Ali's "secret" weapons?