Pulled from my Top Playtime on Steam (so you know you'll get your money's worth if you like them!)
- Factorio (Even though the DLC is pricey, it at least doubles the playtime. Though, like RW below, it takes a lot of content/concepts from fan-made mods.)
- RimWorld (Though I dislike how many DLCs they're pumping out and how much is basically taken from mods.)
- Terraria
- Mount & Blade (Warband 1 & 2 are prime)
- Kerbal Space Program (NOT the sequel)
- TerraTech (The original is a better experience)
- Both the Pathfinder games
- Grim Dawn
- The Riftbreaker
- UnderRail
- Mindustry (Open source/free, but deserves the support from buying it on Steam.)
- No Man's Sky (They've gone above and beyond in shaping up since the clusterf release.)
- The Long Dark
- Rise to Ruins
- Space Pirates and Zombies (1 and 2 are both great, in their own ways.)
- Caves of Qud
- Tales of Maj'Eyal
- STALKER (I've heard bad things about #2, but the rest are great, high replayability with Anomaly.)
- Astroneer
- War for the Overworld
- Autonauts (and Autonauts vs Piratebots!)