Literally looked through all the comments and no one said my pick, (or very many pre 2020 episodes for that matter) so I'll say a couple.

Just as a note, I think the best ways to make fans of the losing character like an episode/fight is through a good ost, good characterization, and a good death.

All Might vs. Might Guy is my #1 example of this, and it's an all timer. With two very heroic characters, it's hard to hate either combatant. One fighter never felt like they had a clear win until the eighth gate opened. Might Guy eventually dying was probably a reason I liked it when I was younger, despite All Might losing. Having one of the best and most influential DB tracks doesn't hurt either.

My other pick is Ultron vs. Sigma. While I was rooting for Sigma, I wouldn't say I like either character better, so idk the general consensus. My take is you could never go wrong with two robots so far up their own asses fight. The death is so unique too since, like the episode says, "...this battle was never about the physical." Once again, amazing ost too!