Waking Mars or Escape Goat would be my choices. I enjoyed both of those. I thought Waking Mars was actually something special due to the emergent ecological systems. I hadn't seen anything quite like it

https://www.gog.com/game/waking_mars $1.49

https://www.gog.com/game/escape_goat $1.29

Pschonauts is also only $0.99. I haven't played it yet, but it's very highly rated
https://www.gog.com/game/psychonauts


Here's a list I used to sort through all the $2 and under games
https://www.cheapshark.com/browse?storeID=6&upperPrice=2


I have Signal Ops in my cart at $1.49, and noticed Star Control 3 is also $1.19, but haven't played either of those

Oh, yeah, how did I overlook Waking Mars when making my post? That's a pretty good game in that price range, too! :)

Quality games that haven't been mentioned yet:
- BloodRayne ~ $1.19
- Cannon Fodder ~ $1.19
- Cannon Fodder 2 ~ $1.19
- Divine Divinity ~ $0.89
- Dungeon Keeper Gold ~ $1.49
- Dungeon Keeper 2 ~ $1.49
- Indiana Jones and the Emperor's Tomb ~ $1.49
- Moto Racer ~ $1.19
- OddWorld: Abe's Oddysee ~ $1.49
- OddWorld: Abe's Exoddus ~ $1.49
- Pirates! Gold Plus ~ $1.49
- Serious Sam: The First Encounter ~ $1.19
- Serious Sam: The Second Encounter ~ $1.19
- Star Wars: Dark Forces ~ $1.49
- Star Wars: Jedi Knight: Dark Forces II ~ $1.49
- System Shock: Enhanced Edition ~ $1.49
- System Shock 2 ~ $1.49
- The Witcher: Enhanced Edition ~ $1.49

My personal pick would be a toss up between The Witcher and System Shock 2.

Double Dragon Trilogy

Two of my favorites:

If you like strategy games: Colonization

If you like adventure: Indiana Jones and the Fate of Atlantis