The game I want the most...
...would be Aliens vs Predator 3. If executed correctly, it could possibly trump Halo in my opinion. With some tweaking of the weapons and a matchmaking setup like Halo 2, I would be in FPS heaven.
Why? I have been an ALIENS fan for 2 decades now. The creature design is intriguing to me, and I just can't get enough. H.R. Giger has one of the most interesting imaginations. So I won't lie...I want to play as an ALIEN. Sure, Master Chief and Arbiter (more-so Arbiter) are solid characters, but they are only based on their agendas and have mostly the same abilities. More options is always good.
If I had a choice between the Marines in Halo and the Colonial Marines, I would take the latter. They just seem to have more firepower. The idea of a pulse rifle over a battle rifle/SMG is already making me want to play it. Out of bullets? There's a grenade launcher on the bottom.... ;)
Ironically, I had AVP and AVP2 for years. Since I'm an ALIENS collector, I actually didn't want to open them. I bit my lip and played them earlier this year. I should have done it sooner, I was disappointed in myself! I missed out when these games were at their peak popularity. So many online games that went on with out my presence...
I did try to get my room mates to play AVP2 multiplayer with me, but unfortunately only 2 of the 7 computers in my house could handle the game effectively (which shows why consoles are more popular). When we had four people it was chaos with all three races running around. It was obvious that multiplayer has grown a lot since this game came out with the jittery movements and hit detection. But...you can temporarily destroy weapons if you don't want someone to grab them...*ahem*...SNIPER!...*cough*.
I can only hope that Microsoft and Fox get together to put AVP3 on the XBox 360. As long as it was put together as well as AVP2 was, I'm confident it would be it hit. Hopefully they stay away from Sony, and get advice from Bungie.