So I've seen this series in passing, as a switch mainly guy where should I start/give the series a try?
On Switch the only options you have are the Senran Kagura spinoff (bad), Super Neptunia RPG (worse), and VII (the best mainline title). Problem with VII is the Switch port sucks, it runs like trash. So... basically nowhere unless you have any other platform options.
My general recommendations for anyone with a similar question are as follows:
Re;Birth 1 (PC) - Basically the easy start point, it's very short and introduces the main characters well. It has not aged well though, and the PC port despite being the best one doesn't even have great controller support. It's regularly on sale for virtually nothing on Steam. Even if you only play it for like a couple of hours to see the core characters (Neptune, Vert, Blanc and Noire) and then dip out due to the aged gameplay, it'll give you an indication of the characters and world is of interest to you.
Re;Birth 2 (PC) - Optional, introduces more core characters and has a more serious story, but it was toned down a lot from Mk.2. Tbh, I find this one best to skip and come back to later if you're really getting into the series. Most Nep games are alternate universe and none really require you to play any previous ones.
Re;Birth 3 (PC) or Victory (PS3) - Not much difference here beyond price and RB3 having some more content. The 3 Re;Birth games all used the engine and assets from Victory (the 3rd game) so this remake barely changes anything meaningful. This is a good one to play, but it is a long one and you might get burnt out. Again, entirely optional and potentially worth coming back to later.
Megadimension Neptunia VII (PC or PS4) - The best of the lot, the only time the series had a proper budget and it made a lot of strides forwards. Minimal asset reuse, darker story, great new characters (especially if you like the Dreamcast), top notch soundtrack by Earthbound Papas, fun gameplay. It's rough around the edges but it's a really fun A tier RPG. If you only play one, make it this. DO NOT BUY VIIR. PERIOD. OK?
Sisters VS Sisters (PC, PS4 or PS5) - Already talked about it above, a return to form after years of bad spinoffs, it isn't essential that you play any others before but it does follow on from VII and so you really should play that first. It falls down in a few areas and has a weak start, but is still one of the best in the series.
Superdimension Neptuna VS Sega Hard Girls (PC) - An oddball spinoff featuring the Sega Hard Girls (personifications of every Sega consoles, they had an anime as well), this one is hard to quantify as it's still kinda like the Re;Birth games but they added some light platforming and action elements. This one kinda shines more from the story and characters than any of the new gameplay elements. Leave this one until you're a few games deep though and invested in Neptunia.
4 Goddesses Online (PC or PS4) - Personally I don't much care for this one, it's a spinoff with action based gameplay kinda like the Sword Art Online games, so if you like those you might enjoy this. Generally it is well regarded among others though, the plot is pretty meandering and there isn't much depth to the combat though (again, kinda like the SAO games) so YMMV.
TL:DR
- If you have a low tolerance for jank then VII followed by Sisters VS.
- If you can stand a bit of jank Re;Birth 1, then VII, then Sisters VS
- If jank doesn't bother you or you start enjoying Neptunia, the three mentioned above andthen pick and choose any others in any order.