One on one battles where a hero has to use his wits or smarts to take down someone more powerful than themselves. Superman vs Wraith has become one of my favorite fights of all time because Snyder wrote a complete badass Superman who doesn't just punches things and wins.
I quite like 1v1 fights, especially as you can generally get a whole lot of focus on a character. Like you have with a Hulk vs Sentry or a Beta Ray Bill vs Stardust fight like Insane Titan mentioned.
But also team battles like Thanos and a certain amount of the Black Order vs the Avengers in Infinity is also awesome as if you get it right you can portray a huge amount of story in it while getting in cool bits of action.
For example you have the pieces in Infinity where Thanos is goading Thor.
I like all types, as long as they're done well (especially those 'underdog' or 'complete ownage' fights). Also, the fight depends on the character. Eg, I like seeing Surfer take on multiple opponents, flying around, blasting things, whereas Superman I prefer in 1 on 1 combat.
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Shinier than a speeding bullet.
I like knock-down, drag-out battles where one or more sides take a serious beating and get severely beaten up in the process, and whether they win or lose you know they gave it their all.
WW Hulk was pretty good. Lots a variety in the characters trying to take Hulk down, and enough time given to most of the fights to make them interesting.
Once the roster gets too big and the fights become brawls, there just isn't enough time or space on the page for the writers/artists to do the confrontations any real justice, so you get dumb outcomes like in Avengers vs JLA, where big names get beaten down in one panel by someone with no business winning like that.