As far as movies and how serious I can take them, the original, 2, and Rocky Balboa in that order.
3 and 4 are great for silly fun, especially 4. I loved it when I was 8 and I remember the theater was rocking when he won; people were actually getting up and cheering. But watching them now, wow it's so unintentionally funny.
5 is just crap; in theory it was a decent idea, having Rocky turn into a trainer and all, but they tried way too hard to basically make it like the original and it didn't work. Tommy Morrison couldn't act his way out of a paper bag and neither could Stallone's kid. The stuff about him losing all his money seemed stupid; I realize celebrities blow their money and file bankruptcy but I had a hard time believing the biggest boxer in the world coming off his biggest victory ever couldn't get a job in the boxing industry making good money. I mean he could have been an analyst on HBO, ESPN, etc. Nope he goes right back to being dirt poor.
Rocky Balboa was a much better end to the series, even though the premise was pretty dumb and somehow his brain damage issue magically disappeared. Milo Whatshisname from Heroes was a much better son than his actual son.