FPS: Unreal tournament, thats right, the first one.
RTS: All C&C love red alert 2 yuri's revenge is the best.
RPG: ES4: Oblivion
Counter-strike Used to be free, before they implomented steam, but im glad they did because i was gonna stab someone after the experience of WON servers. In aus, stupid WON could not find any other aus servers, so at best of times, i was playing with 200+ ping. there were occasionally 8blocksofconnection servers (1.5 was the downtime of my life) but it would never connect to them, i was going insane...then i discovered gamespy, always had fast aus servers...then steam was made, and we were made to pay.
But i have to agree, the HL1 multiplayer kept me entertained for a long time. Oh man i miss those trip mines, some of the best fun in my gaming life.
The sad fact is that i can read script, but i just cant write it.