Honestly any of them are good starting points maybe outside of 6, 7 and 9. This might just be personal taste but I feel like 6 is the weakest in terms of characters, designs and world building. And seeing as gameplay wise almost all of them play the same so that would be a huge turn off. DQ7 I would save for later because the game is HUUUUGE. like I believe if you're just doing story stuff it's just barely over 100 hours, but the character writing and such is really good though. 9 is another game to save for a later play after you get a couple other DQ games under ya. Mostly because the game is just full tot he brim of older DQ references and the game gets really hard really early and assumes you have a good understanding of the series when it comes to mechanics and stuff.
Now good mainline games to start with is 3, 4, 5 and 8. Although 3 is the last game of the Erdrick Trilogy (Don't worry you don't have to play them in any order they're all loosely connected and take place hundreds of years from each other), the game is a nice sample of the DQ series and what will come going forward.The best version is the mobile or Switch ones. 4 is like 3, but instead of having to make your own characters, you have actual characters with predetermined job roles and a pretty decent plot. I recommend the mobile version because the game has the party chat system like you would find in a Tales of game. Every other version of this game doesn't have that and the character writing suffers once their chapter ends, In every other version they pretty much become blank slates that don't react to the plot anymore.
I'm extremely bias when it comes to DQ5 because it's my favorite game in the series, but the game sets up everything with a purpose and then the payoff for them are so damn good it's amazing that a lot of video games haven't been able to pull it off as well. Just a real masterpiece of not only the DQ series, but video games in general. Grab either the DS or mobile version of this one. 8 I would say is pretty up there in the series and is just a real joy on it's own. Great characters, nice world building and a pretty good plot. It's the definition of a cozy, get under the blankets with your favorite snacks and vibe JRPG. Although I think the PS2 version is the better version because of the orchestrated OST, the 3DS is a good time too and adds two new party members and a couple extra dungeons. And this is just an addendum but if you have any current gen console or PC Gamepass, 11 is a really good DQ game too and is probably the best one made to date. I say just give each cover of each mainline game a look at and see which one interests you the most.