DOA4: Definitely my most fun on XBL. Ultimately, knowing equally skilled people and friends is still better than playing strangers, but going into the l337 channel and beating the number #1 ranked player in the world while he talked trash with his friends was priceless. Despite some issues with the ranking and point gain/loss system, this was the definitive XBL fighter.
Mortal Kombat Deception: This is what I played until DOA came out. Largely lag-free and enjoyable enough, but the high end game was so ridiculously broken that it became pointless (although it takes a long time to find those glitches and reach that level of playing). Playing the MKD's version of puzzle fighter was great too. Strangely, this is the only game I was content to waste random strangers with, but the lobby system was pretty trashy like any non-DOAU fighter.
Mechassault: I played this a lot, and it was simple but good fun. Ultimately better again with friends.
Splinter Cell: Pandora Tomorrow: Generally great experience, althought the player limit was a little stifling. Also, extremely steep learning curve. Ultimately better with friends.
Halo 2: Only fun to play with TNL and my friends. Good experience and design.
