This is a weirdly complicated question, even though it seems simple on the surface. The reason is that "series" and "sequel" don't mean to JRPGs what they mean to other genre. The two biggest series of JRPGs - Final Fantasy and Dragon Quest - are more like anthologies than actual sequels. They're almost all self contained stories - even the ones that are directly related to one another.
For instance, the first three Dragon Quest games are all part of the Erdrick Trilogy, but the games don't actually reference each other that much, and you could grab any of them at random and be fine - in which case, I'm going to suggest the recent Dragon Quest 3 remake available on Steam or either the Super Famicom or Gambeboy versions. The original is a bit rough around the edges, but was awesome when it first came out. God, I am old.
For Final Fantasy, the first 10 games are independent stories that are completely unrelated to each other; they don't even take place in the same worlds. (There are two side stories to IV, but they can be safely ignored). 6 is the best of them, 4, 5, 7, 9, and 10 are also good. The older ones are a bit tough to get through from a modern perspective, and IMHO 8 is absolute garbage (I know it has its fans and I'm not here for a fight, just offering an opinion).
Similarly, Chrono Trigger - the greatest JRPG of all time according to many - technically has a sequel, but you can play it or Chrono Cross without reference to each other without issue.
All of that said, I'm going to drop a special mention for Skies of Arcadia. As someone else mentioned, it really is outstanding. I bought the original Dreamcast version on the first day of its US release, and love it to this day. It feels kinda dated in places, but is 100% worth your time.
A more recent one that doesn't hail from Japan is Ikenfell, an indy game that's a wonderful JRPG style game with interesting tactical combat and a good story.
Someone mentioned Crosscode, which is getting a sequel, but it's a single game right now, so you could get in on the ground floor lol. It's a fantastic game, although it's an ARPG so that might not be what you're looking for.
Some other good single installment recent games include Ara Fell, Cosmic Star Heroine, and VOltAge: Genesis.
Have fun!