This is a good question for me as I tend to be super picky about my protagonists! Generally, I'll only like a protagonist that's either really flawed in some way (that isn't just being an edgy loner because I don't like that trope either), or has an incredibly dynamic character arc where they're forced to change a lot over the course of the series—or both at once, as the two can easily go hand-in-hand. I especially like protagonists where they are presented as a typical "shounen hero" but some of the flaws of that archetype are actually addressed and they have to face the consequences of them; Judai from GX and Battler from Umineko immediately come to mind.

But yeah, mostly, I like protagonists that can be wrong and make big mistakes, and protagonists who come out of the series a very different person from how they started.