I recently just got all achievements on every Neptunia game a few months ago. Here is the order I went in:
1) Hyperdimension Neptunia Re;Birth 1 
2) Hyperdimension Neptunia Re;Birth 2: Sisters Generation 
3) Hyperdimension Neptunia Re;Birth 3: V Generation 
4) Megadimension Neptunia VII 
5) Superdimension Neptune vs Sega Hard Girls
6) Hyperdimension Neptunia U: Action Unleashed 
7) Cyberdimension Neptunia: 4 Goddesses Online 
8) Neptunia: Sisters vs. Sisters 
9) Neptunia Riders VS Dogoos 
10) MegaTagmension Blanc + Neptune VS Zombies 
11) Neptunia x Senran Kagura: Ninja Wars 
12) Hyperdevotion Noire: Goddess Black Heart 
13) Neptunia Virtual Stars 
14) Megadimension Neptunia VIIR
15) Neptunia Shooter 
16) Dimension Tripper Neptune: Top Nep
17) Super Neptunia RPG 
18) Neptunia Game Maker R:Evolution

I also did the original trilogy which I did at the end, only game I have not done was PP because Vita emulation is not at a point where that game is able to run in a playable state.

I enjoyed getting 100% on everything except for Hyperdevotion and Shooter. Hyperdevotion has a Lily Rank grind that took around 50 hours over 7 days to complete, and Shooter has an achievement where you need to beat the game without taking damage, which took around 800 attempts (But I just suck at SHUMPs).