It doesn't necessarily have to be bad, just one that you don't think is as good as the fandom makes it out to be.

For me, one of the most overrated battles is Ash vs Paul. It's not bad, but it's not that great. Not only did we see way too much of Infernape, but it dragged on for way too long. Electivire vs Infernape could have been shortened. I don't think it was really worth waiting 2-3 episodes for.

There are other overrated battles, but this is just the first one that comes to mind.