About to finish this up and really enjoyed it, first Atelier i've played since Sophie (played Escha & Logi and Shallie before that) and was looking at the PSN sales ending soon in EU. The main ones I was looking at are Firis, Lydie & Suelle and to a lesser extent Lulua (I only played Totori from Arland) and Nelke (I know this is a spinoff). Any recommendations on what to go for?
Quick caveat: Ryza plays differently from previous Atelier games, mostly because the battle system is real-time but also because alchemy works differently from prior titles (Gust tends to change the system with every trilogy). This probably won't be a big deal for most people but thought it was worth noting.
My recommendation here would be Lydie & Suelle. Prior knowledge of the Mysterious trilogy would be helpful, which you sort of have in Sophie, but it's not necessary. Honestly, Firis feels kind of different here than in her own game anyways, so the biggest thing you'd be missing out on is Lianne not being a one-note caricature. (She's got at least one and a half notes in Firis.)
Lydie & Suelle is my favourite of the Mysterious trilogy for a few reasons: the protagonists are more interesting than in previous games, and I especially like the relationship the two sisters have, where they kind of fill in each other's weaknesses and whatnot. It's simple and obvious, really the most basic way to write a pair of siblings/twins, but it works. I also think the game is better at guiding you to a particular goal than Sophie or Firis were, and for me I think it strikes the best balance of story progression versus openness: there's always something in the story to do if you just want to get on with things, side tasks to perform if you want to take a break, and the usual minmaxing of items and gear if you just want to do alchemy for a few hours.
About Firis: it's the most divisive Atelier game in a while. I think its pacing is terrible and the open-world aspect doesn't work as well as it should, but there's still a solid Atelier game underneath. Definitely still worth playing, but I'd probably save this one for later unless you want to play the Mysterious trilogy in order. And keep in mind when you play that the game will feel like it's missing substantial portions of the Atelier experience (like getting to know your party members and going on side quests for them) for a surprisingly long time.
About Lulua: it's fine. Works better if you have nostalgia for the Arland series, which lots of people have but you probably don't. Not my favourite Atelier game but not bad by any means, I'd probably rank it in the middle somewhere.
About Nelke: I really liked it (more than the other two Atelier games released last year, even) but it's not a traditional Atelier game and works better the more games in the series you've played because of its setup. I did enjoy the characters I hadn't gotten to know yet, so it's not like you can't have fun, but it might be worth saving for later.