Undertale, it was even voted by people as the best game ever made on gamefaqs
The top 5 games on Steam in order:
1. Cave Story +
3. Freedom Planet
4. Metal Gear Rising: Revengeance
5. Killer Is Dead
In my opinion.
Spore ( because it's waaaay cheaper than on origin or store and you can activate the keys from steam on original that way allowing you to mod spore and enjoy retarded fun experience )
pay day 2
valve games in general.