The current one that's out NOW is modern warfare 2, not bad but alot of hardcores play online. The newest that's not out YET is black ops, check on gamespot for more info.
MW2 is fine online if you have friends you can play with. Otherwise it's boring and you end up in games with a bunch of people hooting and hollering if they get more kills than you. I play with some people from work a couple hours a day. I'm not a great player, but it's still fun. 1:2 Killeath ratio, 1:2 Win:Loss ratio, 13% accuracy and 13 max kill streak. I play hardcore most of the time, it's a lot more fun. You do meet some pretty annoying people. Just the other day we were playing Hardcore Headquarters in Bailout. The other team was filled with people with Grenade launchers, claymores, and One Man Army. They just randomly fired grenades down all the hallways, over buildings, etc. Then they started hooting and hollering when they won.
Sounds like my kind of team, but i wouldn't hollar...that's just asking for a head kicking. Hence why mike is off when I play, I rather fist pump XD