For info about GGGnK this is a good place to start: https://gegegenokitaro.fandom.com/wiki/GeGeGe_no_Kitar%C5%8D_Wiki
There are already 7 series if you also count Hakaba Kitaro. There are 1968, 1971, 1985, 1996, 2007, Hakaba and the current 2018 version. Only 1971 is directly a sequel to 1968. Although if you also count Jigoku Hen of the 80s version as separate then that is a sequel of 1985 and that would make the number of series 8 already.
Hakaba Kitaro is unique in that it starts with Kitaro's birth, which makes it a kind of prequel to the other series, and it's the one I started with since it was the only one I could find on a streaming site at the time (not sure if I already knew Nyaa back then). It does explain things like why Kitaro has only one eye and why his father is just an eyeball, but if you read the manga from the beginning I suppose it might be explained in that too (I dunno, I've only read a few stories I could find on the web). It's also a bit darker than the other series and much shorter (only 11 eps).
I then started from the 1968 version and watched up to the 1985 version after which I took a break. I skipped to the 2007 version when the current version was announced, because I thought it was possible that it could be a sequel (it wasn't) and because, I liked its design of Kawauso, which was the character that got me interested in GGGnK in the first place (kind of ironic that I started with Hakaba then, which lacks Kawauso.)
In the end it's all up to you in which order you want to watch. Hakaba is a good place to start because it's short and explains things the other series don't, but 1968 is also a good place to start because you get the classic experience of the original anime adaptation. 2007 is a good one if you like more good natured yokai like Kawauso and Rokurokubi, which are uniquely heavily featured in that series, although the 1985 version also has a nice set of episodes with Kawauso, and the latter half features Shisa, which is also one of my favourite yokai.