In the short run (new few months): Battlefield 2. The built in voicechat and Commander mode can bring the MP FPS genre to a new level, if it's done well (i.e. there's a way to keep the griefers in line).
In the moderate run (late 2005): Call of Duty 2. CoD is still the king of WW2 shooters, with Brothers in Arms turning out to be a disappointing console port, and this one promises to be less of a railroad, allowing multiple approaches to an objective, and taking the AI off simple scripts.
In the long run (2006): NWN2. I still am running three weekly campaign games of NWN, almost three years after the original game came out. That's longetivity (at least for me), and even now, a year or more before release, the Bioware forums (they're hosting them for Obsidian) are amazingly active.
Honorable mentions: Civ4. I still play Civ2 occasionally, but I'm interested to see what Maier has up his sleeve.
I am also a bit interested in XBox360; although I skipped all 3 of the current consoles, I may be buying an HDTV in 2006 and may bundle a next-generation console with it. PC gaming is superior IMO if you can afford a high-end rig, but it's both less social and less suited for a quick, casual pickup game if a couple of friends come over (short of setting up a LAN). I'll be paying close attention to the reviews and the initial release lineup for the new console.
The big words in PC games for the next two years: complex physics, destructable terrain, realistic AIs.
I'm looking forward to an interesting 24 months.