It REALLY depends on the purpose. Someone mentioned GeneratePress and I can confirm (since I've built some very high-demand sites with it) that it offers best in class performance, although it comes with a cost of not having many out of the box features (and rather basic design), which might mean you gotta put some extra work. So it's not always the best choice. If you're after performance, Astra is a great alternative btw., it's got better templates and more features to choose from.
From a design+features AND performance standpoint, look up themes by: Pixelwars, Estudio Patagon, dedalx. I'm sure there's more great creators like that as well, but those are tried and true for me.
For more complex / multipurpose solutions BuddyBoss Theme along with its platform is probably the most seriously developed thing in WP ecosystem right now, with some React implementations to speed things up, sick integrations and what not.