For me it was the original Subnautica. Was a free give away before it got popular and I had no expectations when I played it. Really enjoyed the exploration and the pacing.
Second was Axiom Verge - I got it for free before it became popular but I don't remember how. I bought it when it went to Steam because I wanted to support the creator.
Both are fun exploration sorts of games if you haven't played them. Axiom Verge is a metroidvania. Kinda a weird game, but in a good way.