CS1.6 with those hardcore techno music forced on my PC unless I literally unplugged the speakers.
After that, shitty autofarm macros on ragnarok online for years.
And ofcourse, Openkore
Else Heart.Break()
If you know anything about programming, it has the nicest story/gameplay ever. It captivated me in the middle of shroom experimentation and semester finals.
The experience is divine.