IV:
First Rocky movie i ever saw, loved it back then as a child.... love it even more today as an adult. The storyline is awesome, as is the music soundtrack (heartssssssssss on fireeeeeeeeeeeee!!!), the training montage was fantastic. Today it is one of the most quotable movies in history (my mates have this thing about quoting Stallone movies.... and Rocky IV is probably the most quoted film - aside from First Blood Part II or.... STOP or my mom will shoot! :1orglaugh)
For storyline purposes, i generally like them in this order: I, II, IV, VI, V, III
Like other ordered lists i could flip flop and change them around at any stage as they're not that bad. As much as V was panned, i enjoyed it - Rocky back at his roots and all that sort of stuff.