A difficult question. The truth is discoverability for games is pretty bad once you step away from the major studios and hyped released backed by huge marketing pushes.

That said I think off the bat I'd say Rolling Star by Nul2 Studio. It's a Metroidvania I recently finished off stream that's not super long (took me about 7 hours maybe? though I didn't 100% it) But surprisingly solid. Like, it's reviewing extremely well, but there's only about 180 reviews on steam making it something of an unknown. It'd be nice if more people gave it a look.