which one?

I love Doom3, but sometimes that game scares the hell outta me...

Online gameplay, it depends...

Ghost Recon is the better 'tatical' game as the scenery is done pretty well and plays into your strategy (camo and cover)

Halo 2 is good for straight out action. The only downside is that nowadays you have a lot of kids that run out killing people with the battle sword to boost their stats. Kind of removes any type of strategical element.