The Dragon Quest games are really fun.
I think Dragon Quest V is the most beginner friendly, while also being a good showcase of what to expect from a typical Dragon Quest game.
Dragon Quest VII is my favourite, and is easily the longest.
Dragon Quest XI is the newest one, and probably easiest to get into.
Dragon Quest III is an incredible classic. But you really need to play through Dragon Quest 1 and 2 to really be able to appreciate it. Although while Dragon Quest 1 is fine, and can be finished in an afternoon. Dragon Quest 2 is twice as long, and is easily one of the most frustrating and annoying experiences I've ever had the misfortune to deal with.
I'd personally recommend playing Dragon Quest V first, then if you like it. Play through all of the games starting from the first, and working your way through them.
Use the SNES remakes for 1, 2 and 3, and the DS remakes for 4, 5 and 6. For 7 and 8, I'd recommend the original versions. 9 has never been re-released, so you'll need to play the DS version for that. 10 is an MMO and therefore has to be skipped. And as for 11, make sure to play the Definitive Edition.
They're not that long. It only took me a month to play the first six games in the series... then it took me a month just to finish the seventh game. Seriously, that game is LONG!