My ranking is pretty much:

1. VIII
2. VII
3. VI
4. V
5. III
6. IV
7. II
8. I
9. IX

I know the rankings look kind of weird. I really enjoyed the hell out of 8 because it was the last one to feel like a DQ game.

VII was amazing because of how long and challenging it was. Even without doing sidequests you were still looking at like 70 hours of gameplay. I wish Squenix would get the dicks out of their butts and release the 3DS remake here.

I loved IX when it first came out, but playing it now bores the bejesus out of me, because it's so easy to kill everything. Even the grotto bosses were super easy. All you had to do was get a multihit skill for everyone and get their strength up. Then just do whatever multihit moves you have and everything dies to it. I killed the last boss in like 5 turns doing that and was only level 50 or so. DQVIII got kinda broken too, but not "kill everything in two hits" broken. If they remade this game and made it more challenging, then it would be so much more fun for me.

DQI just didn't age well. That's all that is.