Honestly, if you happen to have access to an old PS2, I'd say start with DQVIII. It's looks pretty, it's still a lot of fun, the voice acting is fairly decent, and it can still be found on the cheap.
Of games available on current Nintendo systems, DQV is my favorite. With the exception of VIII, it has the most developed story and characters of any DQ game, and has a monster recruiting element.
DQIV is unique in that you play as so many different characters over the course of the game, and the character mix changes up significantly in the early parts of the game. The only drawback is that you're constantly leveling up new characters for the first four chapters, and you tend to spend that time fighting the lower-level enemies. If you're into the early game of RPGs, then it's probably for you.
All that being said, I look forward to VI. I've always wanted to play it because of the class system. I haven't gotten far enough into IX to play with that aspect yet.