When I think of the games that really got me hooked, I think of Chrono Trigger and Final Fantasy VI/III for the Super Nintendo--those were the first RPGs I played through, and while there are a few rare gems later on, those rank at the top of my list in the genre. I also loved FF Tactics, and I feel that FF8 is a little underrated in the FF series. But to this day, nothing has matched CT, and given the happy nostalgia factor, I doubt any new RPG can dislodge it from the #1 spot.
When it comes to grand strategy games, I started with a classic DOS masterpiece: Master of Orion. I still play it time to time. It had an absurdly simple planetary management and combat system, which was easy to learn. It really shined in ship customization, though--you get to design and build your own ships! It's a shame Microprose went under... they would have rocked the gaming world today.
I got started on Civ2 as well, played Civ3 and lurked on these boards for awhile, but it was Civ4 that turned me into a dedicated turn-based gamer. I signed up here, started modding, and worked on strategies far more than I have for any other game. In terms of dollars per hour of entertainment, Civ4 is probably the best value on any game I have ever played.
What else... Portal, I think, is one of the most creative games of the last decade, and it makes my top 5 list. It's a first person shooter... with no shooter. Brilliant! I haven't bought Portal 2 yet, largely because I'm now condemned to a 4-year old laptop that doesn't have the ability to play new games.
Finally, my current gaming is shifting to board games (I play a lot now that I have a good and reliable group) and a little bit of Paradox/AGEod stuff (EU3, Victoria 2, HoI3, RoP, BoA2, etc.). They are working their way to the top, but I don't have the history with these games that I have with the others. So they lose the top spots to nostalgia.
So, my top 4 of all time, no particular order:
- Chrono Trigger
- Master of Orion
- Civ4
- Portal
The #5 spot has too many contenders for me to pick from.