For me Rocky works in pairs:I actually thought V was one of the better ones. Didn't really care for 2 & 3 that much.
Rocky I & II are the same movie (underdog stands up against the establishment and proves his worth)
Rocky III & IV are the same movie (go back to your roots and fight for honor once more by avenging the death of a loved one)
Rocky V & VI are the same movie (prove you've still got it but only to a point, you have to make time for your ungrateful kid too)
On that note I'd pick 1, 4 and 6 as the best of each phase. The other movies are expendable. That Rocky wins a rematch against Apollo in 2 is missing the point of 1, and pure unrealistic wish-fulfilment. I think either 3 or 4 works for the "full of himself bourgeois Rocky" phase but 4 is more memorable for the 80s exacerbation. And out of 5 and 6, the latter one is the better movie. 5 suffers from tonal whiplash (and continuity errors), 6 handles the nostalgia way more effectively. You could end 4 with Rocky in the locker room suffering from brain trauma instead of having that open 5,, skip the fifth movie entirely and then catch up to 6 so we don't have to do endure any more pointless fights with Adrian or have Rocky reconnect with his son twice (and apparently a third time in Creed 2, but I haven't seen that one yet).