I'm not really that much of a gamer, meaning I play a lot, but I don't play a lot of different games, though what I play is because it's great. Therefore I'll base this pretty much on time-consumption.
1. Tibia (MMORPG)
I started out playing this back in 2000, yes.. 10 years ago. Basically all of my friends played it, and it was free to download and free to play. It was my first experience with MMOs, I got hooked right away. I played quite intensely the first year, after which a lot of my friends quit and with them I did as well. Though I started back up in 2003 and played on a different world and made a bunch of new friends, amongst others a particularly high amount of Dutch players. After that rush in 2003 I've played it on and off ever since, on a few different servers, but the one I started on in 2003 is still my main server.
Why is Tibia great? The community is important in the game, and thus the chat is a very prominent feature - you cast spells using the chat, you communicate with NPCs using the chat and you communicate with players - there's a lot of different channels as well, like Trade channel, help channel, Guild-Chat, Game-Chat and Personal Messages. The thing is, it's not some tiny transparent text in a corner of your screen, it's a tabbed pane with text on. Furthermore Tibia has no limits; you can reach whatever level you want, train your weaponry skills as high as you want and develop your magic level to new heights. It's 2D, made in 1998 and still going strong. best damn MMORPG out there.
2. Final Fantasy (RPG Series)
I have admittedly not played them all, but I liked every single one that I have played. I am also confident I will enjoy every future instalment of the series.
A great, long story, combined with interesting characters and sub-plots. The combat system is changing from game to game, but it's always enjoyable.
Most people have heard of Final Fantasy, so I won't be adding more details about it.
Final Fantasy XII is probably my favourite thus far though. Even if it was the one I spent the least time on (roughly 100 hours), I liked the characters and the world, but particularly the freedom compared to former games. There was a lot of more quests I could embark on at the time where I finished, but I just rushed into the final boss and defeated him, so I took a break and for some reason haven't picked it back up. Worth mentioning is that Final Fantasy 7 holds the grandest world with the most interesting characters. I mean.. Caith Sith.. HELLOOO?
3. Icewind Dale (RPG Original, Heart of Winter and Sequel)
Unlike the others, the third place was more of a challenge, as there's a lot of awesome games that I've played a lot that deserve this place as well, but Icewind Dale just comes out on top. Why? Well, it's a great old game with an intriguing storyline and I always enjoyed making my own characters. The world is simply charming, and a lot of the NPCs are really peculiar, and that adds a lot to the believability of the plot.
Honourable mentions: Morrowind, Mass Effect, Unreal Tournament, Starcraft, Command & Conquer : Red Alert, Transport Tycoon, Rise of Nations, Soul Calibur, Burnout, Warcraft, Two Worlds, Knights of the Old Republic and Heroes of Might and Magic.