I think one key aspect here for me is that there's more *variety* in The Orange Box - sure, everything's in the
context of an FPS in both titles, but Portal was pretty groundbreaking in establishing a new benchmark for puzzle game design and narrative, and TF2 has its own complete structure that isn't just 'multiplayer shooting in the Source engine'. MCC gives you a ton of
game, but the vast majority of the content is fundamentally samey.
That said, I will admit I'm not really much of a multiplayer person, so while I've played TF2 a bit, I haven't explored Halo multiplayer all that much - there may be a lot more variety in it than I know about.
(All that said, I think I'd probably be inclined to go for neither of those two, given my own tastes. Heck, for me, something like these would be more my flavour:)
(Not *as* much content as the others, but the price at launch was - IIRC, ÂŁ20 - which at that time was
really cheap for how much you got)