Moddb was mentioned. Another good one is thunderstore. It all depends on the game though. Valheim (and several other units based games) is very active on both Nexus and thunderstore, stalker games tend to be moddb, &c. Nexus tends to be the main one for most games though.

I mostly like Nexus (paid member), but I share the concern about it being the only game in town for most games. Nexus is heaps better as a site than both moddb and thunderstore ime, but the lack of real alternatives is putting way too many eggs in the same basket.