Most of what I play is indie and choosing a favorite is too hard, so instead I'll go with biggest playtime. Antimatter Dimensions, also on Steam, has quickly shot to having the highest playtime of my Steam library. It is an idle/incremental game. Bonus points: free! Most of the idle/[email protected] I have played have been free in the browser without IAPs, and seem to have been made by one or a few people.
Not counting that, I'd probably have to go with Stardew Valley.