I own the boxed DVD set of Rocky I-V. In order, the most viewed are:
(1) IV (epic training scenes make this the most fun to watch)
(2) III (80s MTV style filiming and a superior villain make this a great watch)
(3) I (the all time movie classic plus the dramatic fight and training scenes)
(4) II (a little shlocky plus an improbable, inaccurate ending-- i.e., the ref should only have been counting for Creed, with Rocky being ruled a "slip")
(5) V (rarely, if ever, Rocky never gets in the ring, avoid like the plague)