8 is amazing but you're waiting. The one that plays similar, imo, is 6. 4 and 5 are vastly different from others in the series. Here's a quick breakdown for you:
1-3: Considered NES classics. They play very similar to one another but the plots are a bit dull compared to the rest of the series. In the OG you only play as 1 character, in 2 you can have 3, in 3 you can have 4 playable party members at once. 3 is considered the best in the OT but there is an overarching story loosely shared between them all. They play fine but it's kinda like starting from FF1 hoping to get into 15 next year. I wouldn't recommend them to a newcomer unless you don't mind excessive grinding. If you DO get them, get the iOS ports. Cheaper and much less tedious than their NES originals and cheaper than the GBC remake.
4: 4 has a lot more character than 1-3. And is very story driven. Its played in chapters and you dont even play as the main protagonist until like 50% through the story. Basically its a story about a bunch of people who meet up and take down the evil threatening their world. Pretty basic, however the character interactions and national dialects make it very interesting. You get a bunch of characters speaking with Russian, British, Scottish, etc accents. It's also cool to play one chapter and see how another character is affected by the events of the previous chapter without knowing the character that influenced their trajectory.
5: 5 is often cited as the best of the 4-6 trilogy. I like it. But its not my favorite. In it you can pick your wife and catch monsters much like in Pokemon (however a bit more rare). The story has the gimmick of going through the life of the hero. Following him from childhood to parenthood. It's actually cool seeing him, other characters, and the world itself evolve around with him. I really like it but its a bit short.
6: I love 6. Idk why. but it has the gimmick of having a real world and a dream world. It can actually get kind of sad in some parts. To me, its the best of the 4-6 trilogy, however the ending can get super confusing and you could easily get lost and not know what to do if you aren't careful... Its pretty long and is my recommendation if youre playing 8.
9: It's alright. It's better than OG 7. But it suffers from having no talking support characters that were around from 4-8 which makes the game feel lonely compared to other recent titles. It's also a bit more serious. But has a ton of throwbacks to older entries and is probably the longest DQ game yet. It's really fun and the character customization is nice too. Story wise it felt shallow and the world didnt feel as "alive" or "fun" as earlier entries and I didnt like the music as much either. However, it does has visible enemy encounters (no random encounters!) and a TON of side quests. Worth the buy if you can get over some of its flaws.
