Spelunky 2 - $20. One of my all time favorites here. Over 200 hours in, don't regret any of them. I could easily double my playtime without seeing everything.

Dark Souls 1 - $30(?). Back in my teen years on the 360 I bought Dark Souls without knowing anything about it. I played through it with a buddy, passing the controller on death, and we had a blast. That first run transcends money for me, I would pay anything to keep that memory. Recently that same buddy and I replayed the game together and are now reworking our way through the series.

Racingmaybe - $3. This game is amazing. It's a turn based drag racing game with an upgrade system. This is the best driving game I have ever played, far and away. It's a hobby project by a solo developer. At it's $3 price point this game is well worth the money. Seriously, if you're reading this, buy it. Or drop your Steam name and I will gift it to you.

Mount & Blade: Warband - $20. This game is really starting to show its age but I still love it. Endless mod potential gives it tons of replayability.

I could go on forever but these ones came to mind first.