The proof is in the montage.
Thanks to Fathom Information Design, we now know how every single Rocky movie breaks down in terms of the amount of time devoted to dialogue, training, montage, pre-fight, fight and credits sequences.
Dialogue:
As you can see, Rocky and Rocky II rely heavily on dialogue to move the story along, whereas Rocky III and Rocky IV could care less about that sort of thing. As for Rocky V, well, we just don't talk about that one.
Training:
The "training" breakdown is a bit misleading because it doesn't count any training done during a montage — which is obviously the key to most of Rocky Balboa's preparation —but it's still interesting to note the difference between the first and last films.
Montage:
This scientifically proves that Rocky IV is the best film ever. Just look at how much montage there is in that bad boy!