So you want to start with Neptunia series, but you're probably feeling confused or overwhelmed by the number of titles out there. This guide will help you with that.

1.
Hyperdimension Neptunia Re;Birth 1

2.
Hyperdimension Neptunia Re;Birth 2 (aka. Sisters Generation)

3.
Hyperdimension Neptunia Re;Birth 3 (aka. V Generation)

4.
Hyperdimension Neptunia: The Animation - (2013)
It's recommended to watch the anime after Re;birth 3 as a major plot point is spoiled. The anime rehashes mk2/Victory's plots. While mk2 isn't spoiled that much, Victory/re;birth 3 is.

5.
Megadimension Neptunia VII

In any order. But I'd recommend doing main games first, to understand the story.

Hyperdimension Neptunia
Hyperdimension Neptunia mk2
Hyperdimension Neptunia Victory (aka. Ultra Dimension Neptune V)
Megadimension Neptunia VII

Hyperdimension Neptunia Re;Birth1
Hyperdimension Neptunia Re;Birth2 Sisters Generation
Hyperdimension Neptunia Re;Birth3: V Generation
Megadimension Neptunia VIIR (Victory II Realize)

Hyperdimension Neptunia: Producing Perfection
Hyperdevotion Noire: Goddess Black Heart
Hyperdimension Neptunia U: Action Unleashed
MegaTagmension Blanc + Neptune vs Zombies
Superdimension Neptune VS Sega Hard Girls
Cyberdimension Neptunia: 4 Goddesses Online
Super Neptunia RPG
NepNep☆Connect: Chaos Chanpuru
Neptunia Shooter

Hyperdimension Neptunia The App - (Android, iOS)
Neptune Collection - (Android, iOS)
Neptune Kisekae! - (Android)
Neptunia & Friends (aka. Mainichi Compile Heart) - (Android, iOS)

Hyperdimension Neptunia: The Animation - (2013)
TCG: Neptune Unlimited VS - (Physical trading card game)

* Re-Birth seres is a remake of first 3 PS games (Hyperdimension Neptunia; Hyperdimension Neptunia mk2; Hyperdimension Neptunia Victory).
** Megadimension Neptunia VIIR (Victory II Realize) is a remake of Megadimension Neptunia VII.

If you have questions regarding this guide or just got something to say, leave me a comment below.
Contributors are also welcome!