Oh well I didn't think of time out into other activities related to the game. In that case mine would probably be Operation Flashpoint because I used to help run one of the major sites which hosted the official forums before codemasters made their own.
Knights Of The Old Republic would run a close second for much of the same reasons

The Gothic Series. An old friend worked as a coder on the series. He got me hooked. I'm a huge fan!

As promised, here are the pictures of the Nuka Grenade I made. It even has a working pin, as when pulled a small contact 'completes' the circuit.

SYBERIA \o/

I play everything from FPS to Casual (No MMO's though), but above all i love Adventure Games. As a matter of fact my 'all time top 20' is made entirely of Adventures.
I don't act like a 'fanboy' (not registered in forums, not active in the community(s), don't bother people online with my obssesion, etc.), but ask any of my friends and they'll tell you that I'm definitely a MYST/URU fanboy.
-Myst
-Myst Masterpiece Edition (slightly revamped Myst)
-Real Myst (Myst in 3D with a tiny bit of extra content)
-Riven
-Exile
-10th Anniversary DVD (Myst + Riven + Exile)
-Revelations
-End of Ages
-URU, Ages Beyond Myst
-Path of the Shell (URU addon)
-To D'ni (URU addon)
-URU Complete Chronicles (URU + POTS + To D'ni)
Have them all. Love them all.
Riven, to me, is still the greatest adventure game ever made.

Sonic the Hedgehog 2, 3 and Sonic and Knuckles.
Sunk countless hours into playing them over and over again. So much fun.
And I have played the Myst games quite a lot too. Those are probably my big two franchises.

The first three Sonic games have some of the best soundtracks ever written for a video game. Fantastic stuff.

Two Words: Total Annihilation. Especially with the UCP.

Super Metroid
Super Castlevania IV
Doom II
Nocturne
I'm a pretty big fanboy of those games.

Ever since I played it for the first time in 2006 or so, it's unquestionably Star Control II: The Ur-Quan Masters. (Yes, it was mentioned twice already, but what else did you expect from a guy with a Spathi avatar?)
Of course, I'm also rather fond of System Shock 2 and Deus Ex, but it seems like everyone on the Internet knows about those two cult classics. SC2, not so much.

Doom II, no contest. I have it 3 times boxed and once on steam. The game is the closest thing to perfection for me and I can remember playing it for hours upon hours back in the day. I still have the jumbo manual it came with along with the various mods for it.

I think the closest I have ever come to pure fanboyism with a game would be the Silent Hill series. Everything about those games is just pitch perfect (disregarding Origins and Homecoming).
Aside from SH 1-4, I'm also a very big fan of Deus Ex, as you can probably assume from my avatar. Also Ninja Gaiden Black has been responsible for stealing several months of my life in my dogged pursuit of completing Master Ninja.

Thanks, I made some Rad-X tonight. It was great fun.
Edit: I also made the Nuka Grenade a bit rusty to improve its looks.

I love you!