Crisis Core was good for creating a new and decent villain, Genesis, while shedding a little light on Zack.
Dirge of Cerberus actually had a very interesting back story with Chaos and Omega. it was only the over the top melodrama which made it come off as cheesy and emo. Not to mention everytime Rosso the Crimson killed a WRO member I was like pfft, please. These guys are pansies.
Advent Children had very very good graphics at the time which still look great today, and it was cool seeing all the characters in high definition. The movie was kinda pointless, and it had Cloud overcoming a personality trait that didn't rear it's ugly head in the actually game. But something that I did was used ACC, cut out all of the characters gay/emo parts, added in some footage from Crisis Core CG, and the movie wasn't actually half bad.
One last thing, Vincent's boots really bug me when he's not a lego character. I mean, they could even stay pointed and golden, although I would prefer them to be like normal combat type boots, as long as they reduced the size. He looks like a Kingdom Hearts reject with feet that big.