All the DS Dragon Quest games are perfect imo - V especially. IV and VI are also really good. V was my first entry, back on the DS. IX is probably the one that has aged the worst, because it had really nice online co-op on the DS and with the servers now defunct - you'll miss out on that feature entirely.
XI is a fine modern jumping in point if you're unfamiliar with the series and aren't a huge RPG fan.
All the DS Dragon Quest games are perfect imo - V especially. IV and VI are also really good. V was my first entry, back on the DS. IX is probably the one that has aged the worst, because it had really nice online co-op on the DS and with the servers now defunct - you'll miss out on that feature entirely.
XI is a fine modern jumping in point if you're unfamiliar with the series and aren't a huge RPG fan.