I love Dragon Quest and even more I love the Monsters spin-off games on the DS. While I haven't played the other games in the spin-off series, that's mainly due to either being that some of the Monsters games are unlocalised or on a different game platform. I just bought Joker and Joker 2 and I'm playing the sequel first, mainly because i bought it first, but I've already played it for over 80 hours and I now I enjoy DQ even more.
Yeah I first started playing Dragon quest when I picked up DQ V: The Hand of the Heavenly bride and later picked up DQ IX and DQ VI, I have enjoyed them thoroughly because I love Akira Toriyama's Art style, which is very reminscient of his other series, Dragon Ball / Z, He's done a lot of other work to, like fan favourite Chrono Trigger, which I also love and own on DS.
If you'd like to comment and give your opinion on these games, then discuss below:

I'm thinking of picking them up. What do you think of Joker and DQVI?

Well I haven't played the first Joker, but the sequel is great, I definitely recommend that. Dragon Quest VI is awesome too but you might get confused with the storyline at the beginning, however it explains itself as you play along. Great bunch too because I got both of them cheap and they're definitely worth it if you can get your hands on them.

Wow, i just Recruited Estark!!!! He's a Post-game recruitable boss and if you beat him in 10 rounds or less he'll join your team automaticallly. Man, was that the hardest RPG battle i've ever faced? the answer is yes, believe me recruiting monsters gets real hard and you only have chances of getting this type of monster if you have monsters in your party that have buffing moves, items like monster muchies, positive pullers etc and powerful level grinded monsters.
Has anyone achieved this in similar DQ games like DQV? because Estark's present in that game too.

Yes! A thread for me to go on about Dragon Quest. :3
Joker was a good game, but if you havent played it I recommend you skip it and go straight for Joker 2 - it's a much better upgrade and (although it's still not as good as the second one on the GBC) it's a great addition to the series.
DQVI is good as well, but not as good as V in my opinion. VI fixes the small issues from IV, has possibly the longest game time, but it's story is the weakest. Still worth a play if you've got the other ones though. I'd recommend 4,5 and 6 in order, but if you're going to only get one, get 5.