What's the best among the 3? Some say there aren't. Maybe their DLC make the difference?
I never played, and I'll like to buy one, no more.

Because it has Plutia? ;)

Because it isn't as small as Re;Birth 2, it has vastly improved minigame (Stella Dungeon), etc.

The only thing which I see as little flaw, is that they made SP static, so it doesn't change with level up.

Plutia is available as DLC character in Re;Birth 1, probably in 2 as well. I didn't play much of HDNR 3 yet, so I can't evaluate role of Plutia in the story :) I am trying to finish one of Agarests, so Neptunia is in my backlog, I just need more free time :)

Absolutely fair enough. ;)
From what I know, Plutia has a major role in 3, where in Re;Birth 1 she was just additional character with Peachy... and both were quire powerful for that. Especially given that, at that point, you should have access to only one HDD.
And people also disliked 2 cause... Nepgear. They don't like her, though... I, myself, am quite intrigued by the Conquest Ending... in this all 'happy'n'friendly' atmosphere, to pull something like that?

If you're only going to get one - Re;Birth 3 is the best of the lot.
Plutia is DLC character in Re;Birth 1 not in 2 and supporting playable character in 3.
Conquest ending... love it! :P
And Nepgear isn't that bad... dunno why she gets pissed on... but Uni FTW!