My top 5
1- Half-Life - This game got me into gaming and it will be always my nr1 no matter how good games will be in the future. Notable cause i've been like 7y old and i've been afraid of dark cause of barnacles.
2- Doom - Still the best fps game even today. Get Gzdoom, turn off autoaim and enable crosshair and have fun.
3- Fallout 2 - Best rpg imo, still didnt complete cause every time save breaks, doesnt stop me from playing it.
4- Mass Effect 2- First one was good, but after i played 2nd, holy shit it was perfect.
5- To the Moon- - Great story and music, just play it, one of the few games that made me cry.