Best two-man coop game?

[shaleblade]

Which title holds up best with two Material Defenders?

[Sirius]

Do third-party missions factor into this calculation, or just the built-in ones?

[Avder]

There are some major bugs in D3 for coop I think.

In D2 the robots do not behave as they would in single player.

I''m not sure for D1, but that's probably your best bet.

[sigma]

The best levels for two players in co-op is large levels with lots of space in my opinion. For example IceWorld v2

[shaleblade]

I was thinking of the built-in ones, but if you have any good third-party suggestions, I'll definitely hear them.

Also, what're some of the D3 coop bugs & D2 AI differences?

[sigma]

Indeed, some upgraded levels have minor bugs. What kind of bugs do you mean?

[Foil]

Best co-op I've ever played was D3 Reactor Gamma. It was created with co-op in mind, and it's fantastic.

[Avder]

I was thinking of the built-in ones, but if you have any good third-party suggestions, I'll definitely hear them.

Also, what're some of the D3 coop bugs & D2 AI differences?

In D3 there's a whole level that gets skipped if played in coop mode.

In D2 many robots that have special behavior act like robots with standard behavior.

[Lothar]

http://www.enspiar.com/dmdb/viewMission.php?id=988 is quite good in cooperative mode.

[shaleblade]

In D3 there's a whole level that gets skipped if played in coop mode.

In D2 many robots that have special behavior act like robots with standard behavior.

Yikes, that's pretty nasty about D3. Which mission?

And do the D2 AI changes make a noticeable difference?

[Avder]

In D3 there's a whole level that gets skipped if played in coop mode.

In D2 many robots that have special behavior act like robots with standard behavior.

Yikes, that's pretty nasty about D3. Which mission?

And do the D2 AI changes make a noticeable difference?

I think it's one of either level 13 or level 14.

And the D2 robot changes do indeed make a difference. Instead of that little blue guy pulling hit and runs on you all the time, he just attacks like any other robot.

[Sirius]

The main differences I've noticed in D2 co-op have something to do with the fact that one player's machine or another has to "take control" of the AI for that robot. Sometimes you'll have the robots not really dodge what the other players are shooting; they can sometimes sort of get confused and go back and forward (mainly the thief does this)... it's usually pretty close but you get away with things from time to time.
Then there are occasions when robot spawners don't trigger. I'm not entirely sure whether this is a bug in the third-party versions or original, but leaning towards original.
There are also occasional synchronization bugs that cause control panels to fail to work (usually when one player hits it in the other player's view but not their own); these are rare but can stop progress in a level if they happen.

Most people find it's worth it despite that, though - and the reason I brought up third-party missions is that D2 has by far the most of those. For in-built missions, though, D1 might be the smoothest experience; though even then, D2 has an expansion with its own mission, where in D1 you're just getting one.
D3 also has an expansion, though I haven't played it. The "slippery robot syndrome" in the original campaign makes me a bit less excited about it.