For me, it's most likely Team Fortress 2, simply because of the class system. Depending on each situation, you could pick either a Soldier, Scout, or Pyro to capture objectives; a Heavy, Demoman, or Engineer to defend objectives; or a Sniper, Spy, or Medic to support other players in both capturing and defending objectives.
Though, Left 4 Dead also had its own occasion of players working together to survive, from defending each other's backs while surrounded by Infected, to dealing the max amount of damage to single enemies like the Tank and the Charger, to healing and/or reviving each other. Likewise for when players control the Infected, where they have to coordinate and combine each other's abilities to prevent the survivors from reaching the objective.
Left 4 Dead. Why? Because if you go out on your own in L4D you're fucked. You're fucked against the AI and you're especially fucked when you play against other players. You have to constantly watch your team's backs, or lose. In TF2 that is entirely dependant on how good the other team is, not to mention you have spies, scouts and snipers which essentially just do their own thing by picking off enemy players. It's only really the demos, medics, engineers, soliders, pyros (kind of) and heavies that have to work together, and like I said, they only really have to work together if you're playing against a good team.
As you said, Team Fortress 2 has each player taking on a certain role, and players depend on each other to succeed - which is what team work is about. In L4D everyone just need to stay close and keep shooting - they wouldn't be able to do it alone, but they don't need each other to function, they need each other for firepower.
Probably Left4Dead, because using teamwork and sticking together with allies is pretty much essential, for the survivors and infected alike. Teamwork is still essential in Team Fortress 2 against any competent teams, but you can still do well by playing as a lone wolf in most less serious matches.
In Left4Dead, the group sort of becomes a single entity, much more powerful than any one of its members put together, and good teamwork will result in this entity having hive mind qualities. In Team Fortress 2 on the other hand, its more about each individual player doing their own thing as best as they can which combines well with each other.
While TF2 has the complementary class system, most of the time it's every man for himself.
In L4D2, at least I get to team up with my friends and we have to work together to get through the levels.
TF2 is like a lot of multiplayer shooters in that a good player can get to a good score completely alone, a really good player can carry an average team to an extent as well (but it's not quite Counter Strike in this respect).
Left 4 Dead punishes diverging from the team more or less instantly, whilst there are some solo runs kicking about on the web they're very much a rarity.
L4D needs teamwork to play at all, whereas TF2 doesn't but they both ramp up and need it fairly equally if you've got a good set of people on the server. So initially it'd be L4D, but it eventually gets to where it's about even, at least in my experience.
TF2 used to be entirely team-focused, now people don't even play capture points or flags anymore - it's all about the battle. They quickly fall into deathmatches so it's not about team-work anymore. At least that's what I get on my servers. It's actually quite tragic, but when when you actually play the maps and people hate you for it.
L4D? If the four of you aren't on the ball, you'll die. You can't even pair off; it's constantly designed to test four people.
Apples and oranges, which fruit is healthier for your eyes?
You can't really compare TF2 and L4D, one's a class-based FPS, and the other is a FPS CO-OP horror.
In TF2, teamwork is highly recommended.
In L4D, teamwork is mandatory. The nature of the special infected makes soloing a campaign at least incredibly difficult. You need even the dumbest of AI simply because they can knock off hunters & kill smokers.
Teamwork? as in I have to rely on you shitty people? no wonder why I never win