1) Fallout 2 - Holy shit, talk about freedom to do anything you wanted! This RPG was all about freedom of choice, exploring several hundred different storylines, and the ability to blast a group of children with a grenade. You were given the choice to kill anyone and anything instead of solving quests. You could steal from people, you could use your high intelligence or high charisma stats and convince someone to do something. You had the choice to either build your character physically or mentally and use them to your advantage to solve different quests. If you didn't want to go through the main quest (saving your tribe by finding the G.E.C.K.) then you could just fiddle around New Reno, take hits of Jet (kinda like cocaine) and screw hookers all night. You could become a boxer, a hubologist (like a scientologist), a slaver, a "made man" for the different gangs in New Reno, and so on...the list can go on forever. This game, simply put, was THE SHIT.
2) Half-Life - I really don't need to go into further detail. There is no game like this and there still never has been.
3)Deus Ex - This was a futuristic first person shooter with a great storyline and the ability to choose dialog and go on different quests. Everything about this game was fucking great.
4) Hitman - Apparently they are turning this into a movie. I have a good feeling this movie is going to suck the balls of a sweaty horse. The whole premise of this game was awesome...you just play a hitman guy that runs around choking people with piano wires, taking their clothes and using them as disguises. You set bombs, traps, and sniped people.
5) Planescape Torment - Another great RPG with a GREAT STORYLINE. Your character wakes up on a slab in a morturary with no idea of who he is. A floating skull named Murray helps you out throughout the game to help you discover who you are. It turns out that you've had multiple personalities...some of your past selves were really brutal mean motherfuckers, and others were really nice and generous. Once again, you're given lots and lots of choices. You could be a mage and be really good with spells and crap, or be more of a melee combat guy. Everything was up to you. Honorable Mentions go out to: GTA(the original), Baldur's Gate 2, Metal Gear, Starcraft and Warcraft, Sanitarium, Monkey Island, Grim Fandango, Doom, Wolfenstein, and the Zork Series..and I can't think of anymore but they'll come to me..