Well as a big fan of the Rocky series, I always stick up for Rocky 5. I am more of a fan of the storyline ones as opposed to the "all american, all action" ones (namely 3 and 4).
There is a hint of realism with Rocky 5, even though the fight scene naturally wouldn't happen off course fighters have lost their money. I thought the story was excellent.
Rocky 4 was just another no retreat, no surrender... and while Rocky 3 was a good action flick - it was a bit lightweight incomparison to 1 and 5.