From what I have heard, Go Nagai was more influential than actually a good mangaka. Like, if you're into the history of manga/anime his stuff is a must read to fully understand the impact on the mediums of print/animation in Japan. But quality wise he's good not great.
I'm sure some will disagree that's just what I've heard, not necessarily my opinion because I haven't read any Go Nagai.
Most of the of his stuff falls into one of three categories.
Over-the-top gag comedies: This is where stuff like Gakuen Taikutsu Otoko and Kekko Kamen falls. Lots of these feature sex jokes and parodies of authority figures (after his first major work in Shonen Jump ended after it became a boogeyman for parent-teacher associations Go Nagai would make teachers a frequent target in his works). I would highly recommend Gakuen Taikutsu Otoko, it's absolutely insane and really funny. For animated versions of these I would recommend the Enma-kun TV anime from a few years back. Kekko Kamen has an OVA that's really dumb but I personally liked it, wouldn't readily recommend it though.
More standard hero archetype stories: Cutie Honey and most of his giant robot work. Sometimes you get some gag comedy mixed in (like Cutie Honey's sex comedy stuff) or some darker stuff mixed in (like the part in Cutie Honey where Sister Jill burns Natsuko to death) but I would consider these his more "straightforward" works. "Straightforward" in the sense that they still feature things like
Nazi Dullahansbut still. For animated versions I would recommend any version of Cutie Honey other than Flash, Mazinkaiser and Shin Mazinger, or Kotetsushin Jeeg. Getter Robo is Ken Ishikawa's work more than Go Nagai's but I'd recommend any of the OVAs.
Dark stuff: Devilman Lady, Dokuro no Yakata, Violence Jack and so on. Often full of sex and violence with darker plots and bleak endings. Devilman sort of falls into this too although it's more restrained than some of his later stuff. These are the ones that he's probably most known for due to the Devilman OVAs not shying away from violence and the ultraviolent Violence Jack OVAs that amped up the violent content from the original manga. If you can stomach the content there's some great stuff to be found in here though. Violence Jack has its reputation, but there are some genuinely great arcs in there. Devilman Lady has lots (LOTS) of rape but there are some really great bits too. For good animated versions of this kind of stuff there's Devilman Crybaby (obviously), the 1980s/90s Devilman OVAs, and the Devilman Lady TV anime (which is very different than the manga). I personally like the Hell's Wind Violence Jack OVA too, but it's not for everyone.
Then there's some stuff that doesn't really fall into any of these
"Prestige" work: Straightforward "serious" works. His straight adaptation of the Divine Comedy in manga form is pretty good.
Weirdo shit: Pure, absolute, unrestrained id to an even greater extent than the rest of his works (which I'd call "barely restrained" at worst). The genderbent version of Enma-kun is probably the grossest thing I've ever read and I have no idea what the audience for it is supposed to be.
As for if any of this is "good"...eh, it's a matter of taste really. I liked most of the works I've listed here outside of the genderbent Enma-kun one, which I'm a little fascinated by even if I find it disgusting. I would readily recommend the anime adaptations I listed more than any of the manga other than Gakuen Taikutsu Otoko (which has no anime).
...this reminds me, I really oughta check out Susanoo sometime.