LogFAQs > #973176820

The first games of any of them are the best to start with.

If I have to pick one my recommendation for first game to start with is Atelier Ayesha. It strikes the best balance between gameplay, plot, and fun characters to give a good first impression of the series.

Ryza 1 would be my second choice for similar reasons if you prefer that for whatever reason, though I don't personally prefer the Ryza series' more actiony gameplay to the turn based stuff.

Rorona Plus is also good to start with (its what I started with) but I do feel as an individual game its not as strong as the other two I mentioned (even if I do like Arland and its cast probably the most of the 4 series).

I don't particularly care for the Mysterious series in general. I wouldn't say they're awful really, but they're not remarkable in any way and are just IMO consistently below the other 3 modern Atelier series in quality. Though I will say I haven't played Sophie 2 yet and I've heard that's quite a bit better.