I'm voting for Marvel Team-Up. Sure, the stories were extremely formulaic right from the get go, but as a dyed-in-the-wool Spidey fan, this is the team-up book I've read most of. I would say, however, that the dozen or so issues of Marvel Two-In-One that I've read make it a very close second.
I've never read Brave And The Bold (I'm not much of a DC guy), but seeing all the love that it's getting in this thread makes me curious to sample it. Is there a Showcase Presents TPB of it available? If so, I may have to take a look.