I guess Utopia on Intellivision got me into world building types of games, that continued with SimCity and Civilization II. Although I've enjoyed JRPGs, I've always preferred western RPGs. After watching my neighbor play The Bard's Tale, I purchased The Bard's Tale II, though I never finished it. My neighbor's love of AD&D got me into the Pool of Radiance game which I finished a few times. Wasteland is another great RPG on the C64 that I played through a couple times.
Starsiege Tribes got me into online gaming, and then I played Tribes 2 probably more than any other game ever
Those two games make it very difficult to enjoy most other online shooters, there's just no comparison with the freedom of movement and large maps. Dynamix was ahead of everyone else when it game to online shooters. T2 had a built-in community with news, email, IRC, team pages for recruiting and getting tags, in-game demo recording, and one of the best server browsers I've come across. They made an attempt at in-game voice chat too, though that didn't work out so well they were thinking ahead. I did later play a decent amount of Battlefield 2, and had tried BF 1942. There are some similarities, but I need a jetpack and spinfusor when I game
I've played some Tribes Vengeance, and am now dabbling in Tribes Ascend, though neither is as good as the Dynamix games.
Everyone's just making lists and here I am going on and on