As a Dragon Quest Fanatic, I can see a reasonable case being made for these games:

3, 4, 5, maaaaaaybe 6, 8

It just depends on what you're looking for. And, to some extent, which version of the game you've played. I don't really think any other answer is reasonable. In my opinion, the "Best" game of that group is the fifth one. However, my actual favorite one is Dragon Quest 4.