Full experience, go in order. 1, then 2, then Nier, then 3. But 3 has the best controls and graphics, and isn't nearly as sad as the other three, and it is set before the others but isn't really a prequel as it doesn't necessarily tie in, so you are free to start with that one or end with it (personally, I'd start with it).
Incidentally, Korra's personality is based on a character from Nier named Kainé.
I also think of Kainé's voice whenever she talks, so think of this voice whenever she talks:
Voice actress is Laura Bailey, by the way, voice of Lust (FMA), Amber (Darker than Black), Schrodinger (Hellsing: Ultimate), Shin (Shin Chan), Keiko (Yu YU Hakusho), Lucina (Fire Emblem: Awakening), Serah (FFXIII), Rise Kujikawa (Persona 4), The Cloud of Darkness (FF Dissidia), Rayne (BloodRayne), Angela (Silent Hill 2), and young Chi-Chi, young Trunks, and young Dende (DBZ). One of my favorite voice actresses.