Portal 2 was fine on xbox 360. Enough said.

Aw, as soon as I posted that I realized someone was going to correct me, but I decided not to bother editing to avoid my shame. But yes, Portal 2 was fine.

Team Fortress 2 on the other hand, was an awful implementation that represents everything wrong with Microsoft's pricing policy.

I'll grant you I've seen WEIRD things happen. Like an AI shooting me in the head. And this one time where the AI insisted I needed to be healed while we were both getting attacked. It's not perfect, but it's better than most companion AI I've seen. (Although some of the best I've seen was in Republic Commando).

I'd go with KF too. I've played KF, L4D and L4D2. All 3 are very arcade-like zombie slaughterfests. L4D1 was great, I loved it; the map designs were ace. L4D2's maps were awful, to say the least. Too long, too tedious, and there were some good ideas, but then if L4D1 had some semblance of survival horror to it, L4D2 ripped most of it out of the picture.

As for Killing Floor, it's one of those games where it's always fun to try out new maps. The pacing is better than L4D2's, and sometimes you just gotta run, whereas in L4D2, you can roughly predict whether a situation's outcome will be plausible or not. Like, you know, when you're playing on Expert and a Tank approaches you and none of you have a single jack of a proper gun or molotov. Especially a molotov.

The "director" also can do some hilarious things with spawning hordes. I once had a horde spawn inside a bathroom which I had just left not 5 seconds earlier. A windowless bathroom. You'd think I would have noticed the 15+ dudes when I went in there. I've also seen at least half of a horde crawling out of an inch-deep puddle in the middle of a road. Fun times.

Why do you want it for the 360? My motherboard is from around 2001, the processor is a Pentium 4, and I have a 512MB AGP video card, but I can play both Left 4 Dead games flawlessly. That's the Source Engine for ya. Aside from Team Fortress 2, Portal 2, and some of the other more recent stuff, everything from Half-Life 2 to Garry's Mod runs great on weaker hardware.

Unless you have a very good reason to want to play it on the 360, a very good reason, I'd suggest getting it on the PC or not at all. I liked the first one more. I liked the levels and main cast more. A sequel wasn't really necessary, especially so soon after the original. More people probably play the second one. Frankly, I don't really know the score at this point; I got bored of the both of them forever ago. Loathed Killing Floor.

Left 4 Dead: Fun and fast-paced. The friendly AI is very good. The difficulty levels do what they say. Easy is easy. Normal is satisfying. Advanced can get tough, especially in the later levels. Expert will make you work hard for success, especially since the friendly AI starts to show some of its inadequacies (though that doesn't mean it gets bad; you just might notice some of the not-so-good aspects).

The levels are awesome and the characters are even better. Weapon selection is minimal. You can duel wield pistols and you have access to a sub machine gun, a pump-action shotgun (the two Tier 1 weapons), an automatic shotgun, an M16, and a hunting rifle (The Tier 2 weapons). They are all effective in their own way.

Left 4 Dead 2: More numerous and more difficult enemies and a few more Special Infected. Possibly the game's greatest downfall is its slightly inferior friendly AI. Again, it isn't bad, and it'll get you through the Easy and Normal difficulties. However, don't expect much help in Advanced or Expert, especially if you need to get somewhere fast. The friendly AI tends to lag behind, and with the increased difficulty, you might find yourself alone with no hope of friendly aid.

Personally, I thought the levels were just as good as those in the first game. The characters, however, are somewhat lacking. Rochelle doesn't really have much personality. Coach is okay in that he's the older guy of the group, and he's good for the occasional joke. Nick has personality, but it really only shows when he's engaged in friendly banter. Ellis, though, is awesome. He's got the most personality, and he's got some hilarious stories.

Weapons are more numerous, even if their functions vary only slightly. The inclusion of melee weapons is a nice touch, I think, though on the higher difficulties you might regret picking one up, since it takes the place of your side arm.

So yeah. That was more... disjointed than I had thought it would be, and it doesn't included all the differences present. But I hope I kind of gave you an idea. Both play almost identically, and they're both a lot of fun. I've never found them boring. Of course, I don't play them for weeks and weeks. Still, I think they are fun games.