>>803526
Sorry I'm a month late but it may be interesting to others. Anyway, Re;birth 1 is the best intro to the series and it's a generally light-hearted game. Re;birth 2 gets more serious (and flat-out hilariously grimdark if you get the conquest ending), and it's also harder so you'd want to be better versed in how the game plays, since it plays the same as the first and the series on the whole is bad at explaining key features.
If you get them, avoid using any DLC characters. They are generally imbalanced. For example in Re;birth 1, Plutia is basically your first three girls rolled into one, while the ones in Re;birth 3 tend to have very cheap special moves (Cave is top-tier). The majority are playable in other games in a more balanced fashion anyway.
Both Re;birth 2 and 3 have retranslations for PC and Vita. You want them. The first game's translation is adequate but 2 and 3 are NISA being NISA. Most of the spin-offs have crap translations too, thankfully most of those are not that great, only check them if you really end up loving the series. Also it should be said that Re;birth 1 and 2 are actually remakes. I've never played the originals but apparently they have atrocious gameplay.