I have to go with The Outsiders here.
The Hart Foundation, while Bret and Anvil had great chemistry, it was still a one man team as far as I'm concerned. I mean, honestly... Jim Neidhart was nothing more than a comedic relief, really. He did nothing but play with his silly beard, and throw punches and kicks every now and then, until finally holding someone up for three seconds for Bret to knock their heads off. That's all he contributed to the team when you look at it.
The Outsiders, on the other hand, was most definitely a two man team. Both Nash and Hall brought tremendous things to the table, and both were equal in The Outsiders success/greatness. From charisma, promos, work-rate... they were on an even playing field when you put it all together. You absolutely cannot say the same for The Hart Foundation.
Now, kayfabe wise... while Bret holds singles victories over both Nash and Hall, it took a LOT for Bret to get those wins, more so than a lot of other opponents he would face throughout his glorious career. To face both of them at the same time, with only the help from someone who was completely unproven against top talent like Jim Neidhart, I just don't see how Bret would have ever been able to pull this miracle off.
To me, The Outsiders win this one in every possible way imaginable.