All of them is my go-to response as well! But specifically 3 and the Zenithian Trilogy (4-6), with special notice to 5 for being crazy good story wise, 3 for feeling really good and open, and 4 (as I'm finding out currently) as having some of the best character personalities in the series, when played on mobile since it has party chat.
in fact, 3-6 are great on mobile, since if you get used to the Portrait Mode style, they look great and have stuff like Auto-saves after each fight and quick loadtimes, though the DS versions are still great.
1 is not bad, but it is a very chill grinding game, about making incremental progress to success, and 2 is a brutal sequel to that philosophy, expanding it.
7 is good, and actally has a ton of value in it's immense amount of content, but the pacing is very drawn out, so if you aren't taking it like a 200 episode anime you are catching up on, I.E taking your time or making big leaps at once, it's gonna feel weird. Still very much a good game, just don't be afraid to look at a guide if you get stuck somewhere along the way : P