For me it was GL-Quake. I was a huge fan of Quake and it ran (barely) on my 486. My cousin had a pentium 166 with an expensive graphics card (S3 - I think). Quake rendered using Open GL was a thing of beauty. That was my "I must upgrade my computer now" moment. I rarely play first person perspective games nowadays but back in the 90's Quake was my benchmark. I never really got into Half Life I loved Unreal but Quake was the real classic.