GTA V definitely.
Mafia II was just too short.
Was alright, but felt short for me.
The Orange Box is not worth purchasing if you already have all of them bar Episode 1.
Which personally, I found to be a bit boring, compared to Half-Life 2 and Episode 2.
Saints Row 3 is a lot of fun, and is worth playing a few times, definitely.
But, no matter how fun it is, I don't think it will hold up to what GTA V has to offer.
You'll be waiting for a bit, obviously, but it's much better to wait for it's release than to waste your money on the others.