The only DQ I ever played was DQ: Sentinels of the Starry Skies. I LOVE THAT GAME!!!!!!!!
It's a great game. A lot of fans hate on it because it's not DQVIII Part 2. I love it. I mean, I have complaints, but it's a great game in it's own right. I imported it, spent 200+ hours on it, and then got the US version (won it in Nintendo's pre-release contest along with a DSi). Spent another 200+ hours. My friends got it and we had a lot of fun doing multiplayer.
My thoughts on each game (because I never get tired of ranking things).
1 - VII: Great somber tone. Probably the grimmest DQ. Pretty awesome episodic story with great town vignettes
2 - III: Set the standard for DQ. Established the tone and nailed the gameplay formula.
3 - IV: This used to be my favorite, but I'm not going to lie, the DS version lowered my ranking because of how god awful the translation is. I really should get the Cell version though. My username is taken from this game.
4: VI: People don't like this game. I don't know why. It deserved a better remake than it got, but it's still amazing game in it's own right. SOme really cool character designs and neat story.
5: IX: My only complaint with the game is that the main story is a bit too easy for my taste. But the game play here is fantastic. An absolute joy to play. Great main story IMHO with a cool final boss. Has my favorite normal battle theme in the franchise
6: V: A fan favorite. I think it's a bit overrated. Don't get me wrong, it has a great main story, but I think it suffers in the town vignette stories, which is more important to me.
7: VIII: Hipster Saro coming incoming. This is the FFVII of DQ. Popular because it was the prettiest game in the series and a lot of people's first DQ. To me, while it is certainly an amazing looking game, it felt like a downgrade after DQVII both in terms of stories told and also game play. Plus... it introduced my two least favorite things about recent DQs: tension and the alchemy pot. If it sounds like I don't like DQVIII though, trust me, I do. I love every DQ. Yangus and Trode are amazing characters, and this game has the best final boss theme in the series (the final boss design is whack though).
I and II are about tied for me. To me, they are sort of prototypes. They didn't quite have the formula down, but they go a long way in establishing the series. I've played both games many times and love them. But I can't say I'd recommend either to first time players.
X... I haven't played enough of it to really rank it, bu what I played of it was great. It's like an even bigger version of IX. I can't say I'm fond of the zoned world, but I guess that's the normal now