I voted Rocky Balboa. Rocky is a classic film and a great movie, but for me, Rocky Balboa took everything Sly got right with Rocky, and the awesome fight elements of 3 and 4 and combined them all with his aged wisdom into one super movie full of a lot of heart, a lot of character development and growth, paid off with an amazing fight.
I love the whole series, but you could also easily make a trilogy out of 1, 2 and 6, they all tie together, 3-5 really have little bearing on where things end up in 6, you could have 6 start off where it does without seeing the events of 5.
Out of the whole series, my favorite action fight is Rocky 4, but my favorite emotional fight is a tie between 2 and 6, I just feel like Sly really hit his stride with those two fights and how to not only entertain you with the punches but draw you in with the struggles Rocky goes through internally and externally. I love in the Rocky Balboa fight when he almost gets knocked out then gets back up and waves his gloves at Dixon like "bring it" and then just goes off on him.
What I love most about the Rocky character is what he stands for, how he's a symbol for when life metaphorically or literally kicks your ***, you have to get up and keep fighting for what you're after.
And I don't care if it's cliche, my absolute most favorite moment in the whole series is "Yo Adrian, I did it!" To this day I still cry every time he gives that speech at the end of 2. Say what you want about Stallone's acting/writing/directing, the man knows how to get you invested in a character and tug at the heart strings, and this moment personifies that most of all for me.
While Rambo is my absolute favoirte character of all, Rocky is my biggest inspiration in life, and I'm so grateful to Sly for creating the character and take him to all the places he did.