I think it's a testament to the series's formula that DQ8 managed to keep charming me despite how dark it got. There's a late game twist with a character that I still think about from time to time.
DQ5 is my favorite in the series by a mile--it's also rather popular in general--so that's an easy recommend for people that like the series. There's also Chrono Trigger if you haven't played it yet, as another in the lighthearted/JRPG/Akira Toriyama vein.