Muck,
You fall into the category of simulation gamer. Stuff like Halo 2, Doom 3, Half-Life 2, etc. aren't going to be your cup of tea
I'd suggest taking a look at The Sims (don't let people fool you, it can be very addicting).. uhm, Evil Genuis just came out, it looks awesome. Check out the Roller Coaster Tycoon series, there's alot of strategy, management, and some great challenges in it. Plus.. you get to build freakin' rollercoasters. I've heard really good things about Rome: Total War, but haven't played it myself. The companies previous titles have been good thought. Warhammer 40k: Rites of War is awesome if you enjoy Warcraft-ish type games... okay so I was at Best Buy yesterday and now I've got a Christmas shopping list in my head...
My favorite games of all time:
All of the id Software titles (from Doom to Quake to Doom 3)
All of the Blizzard titles (from warcraft 1 to world of warcraft)
Counter-Strike
Call of Duty (short, but blows away the medal of honor games)
American McGee's Alice
The Sims!
RollerCoaster Tycoon 1, 2, & 3
City of Heroes (best mmorpg out right now imho)
Aces High II (didn't play 1, and this is the only flight sim I play)
The downside is you can really only play 1 massive online game at a time. Well, at least I can't play more than 1 anyways. Splitting your time between the two makes scores suffer & tangible skills slide away