I found that farmland Saga was probably up there as one of my favorite arcs of the series just in seeing how Thorfinn grew as a person. It reminded me a lot of the similar arc that Vagabond went through which also added a lot more depth to Musashi's character.
Funny thing is, once I finished up farmland saga all satisfied I had a hard time reading any further in the story and just kind of dropped it. I found myself not really caring that much and it became somewhat too humor filled, got some tonal whiplash.
Probably your best bet is reading through the original manga as there isn't really a complete anime adaption that properly adapts the whole thing. The original series from the 70s is nothing like the manga or the rest of the series as a whole, if you want anime I'd recommend the two Devilman OVAs as the best we'll probably ever get (If you want cheesy goodness watch the dub for at least the first one, but if you want it serious watch it subbed) and it features a great ost.
https://youtube.com/watch?v=ymWc_AxitpYThere's also
Amon: The Apocalypse of Devilman which I haven't personally seen but haven't heard very much about besides the ridiculous levels of gore. If you end up liking Amon you can check out the
Violence Jack OVA series, full of sex and gore, which technically connects to the Devilman series.
Basically, you can read the original manga and from there just pick what looks interesting, there's numerous spin-off stories that make use of the setting or characters from the original. For example,
Devilman vs. Hades takes place after the end of the series and requires some knowledge of the Mazinger Z series while
Devilman G is a manga that very loosely follows events of the original while featuring its own characters and plot. Mangadex's breakdown can kind of give you an idea of what I'm talking about, even if it lists Devilman Lady (which I've heard some people claim the anime is actually better) and Demon Knight as prequels you don't need to read them. There's a lot of collaborating with other Go Nagai titles or with stuff like
Cyborg 009 vs Devilman it's because Go Nagai wanted to do a collab with Shotaro Ishinomori (creator of Cyborg 009 and the guy that Go Nagai worked for as an assistant before making his debut) in some way.
None of it is needed to enjoy Devilman, they're just nice bonuses.
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If anyone claims that Crybaby is a good starting point don't believe them, I would consider it more a spin-off/Alternative story with the amount of liberties it takes. It also messes up a lot of things due to this, probably the worst adaption of Ryo.