It doesn't really matter which one you start with. The series is split into trilogies which are all completely separate from one another, but even starting in the middle is fine. Ryza or Sophie are the generic recommendations.
Arland (Rorona, Totori, Meruru, Lulua) is the only one with somewhat strict time limits. Great if you want involved, challenging gameplay, not great if you want something chill. Totori in particular is hard, and as such probably the worst place to start. It also has a bunch of fanservice, and the trilogy as a whole also features the worst character in the entirety of the series, so it's not so great if you want a good story. They're also pretty bad PC ports. Totori stutters on menus and randomly crashes, and Meruru has an avoidable crash on the final boss fight (basically it'll chrash unless you play in windowed mode). Lulua is pretty much just for fans of the trilogy.
Dusk (Ayesha, Escha & Logy, Shallie) has great art, and very lenient time limits. It's the one trilogy I haven't really played, so I can only say that Escha & Logy is incredibly easy. I started with it because people recommended it, but was pretty bored by the lack of challenge. Still, if you're not looking for a challenge, they're a good place to start.
Mysterious (Sophie, Firis, Lydie & Suelle, Sophie 2) is pretty inconsistent. Sophie is good, probably the most solid on the gameplay front, but the story is just okay. Firis is kind of a mess. Gust tried to make an open world Atelier game, but didn't have the resources to pull it off. Widely ambitious by series standards, but also very janky. Not great as a starting point, but I like it for trying something new. Lydie & Suelle plays the most like a straightforward JRPG. Haven't played much of Sophie 2, but it seems like it doesn't really require any knowledge of 1. The PC port of Sophie 2 runs worse than Elden Ring so beware if you have a weak PC.
Ryza (1 and 2) is pretty much just thighs. Well, if you're fine with the character designs, they're actually pretty decent games (and other than the character designs don't have as much fanservice as some of the other games in the franchise). They have great production values, and a somewhat interesting real time combat system. Ryza 2 is incredibly easy due to changes in the way the crafting system works.
Also just in case: the more recent Atelier PC ports will stutter heavily unless you plug in a controller.