Hajime no Ippo was an immensely popular manga in the 90's and beyond, the anime adaptation is not even that old and it was again really popular. About 8-10 ish years ago Hajime no Ippo exploded in popularity in the west and is generally on a ton of people's top lists for manga/anime.
Dragon Ball of course is not as popular as DBZ, so by that comparison I can see that it might be underrated. But it is still incredibly well known with massive brand recognition.
Area 88 I'll give you as not many people are that familiar with it and it likely doesn't get the appreciation it deserves.
Again sorry if it seems like I'm attacking the OP, it's just that this is a very good thread premise but then to list two out of your 3 initial choices as 2 of the most popular/well known manga of all time with popular well liked anime adaptations seems really weird to me.
For underrated I would say the 90's Giant Robo OVAs. Absolutely phenomenal series. Pretty much pioneered the modern "retro" look, great animation and action sequences. The music is absolutely incredible, each OVA has it's own complete score composed by Masamichi Amano. The storyline is great with a lot of unexpected twists and there is some great melodrama in there. Funny enough when creating Giant Robo for some reason the director was not allowed use the original characters/story from the manga other than Giant Robo himself (and I assume Daisaku). What the director did then was ask could he use characters from the managa creators other famous works (Babel II etc..) so he created an almost amalgam world with characters spread across his works, pretty much a love letter to his work.
I think a lot of people were put off by the cover art/general art style at the time because mecha anime with sexy designs were all the rage and here we have this hulking Egyptian looking robot on the cover.
EDIT: Almost forgot, the late 80's/early 90's Devilman OVA is amazing. Much much better than Crybaby. It was reasonably popular at the time of release but kind of faded from memory as anime fans became the type who want 2D one note archetypes, over done unoriginal 'skits' and 9 year old girls. To them Devilman was just this weird violence porn nonsense. To this day it's still one of my favourite anime and I think it stays true to the spirit of the manga much more than Crybaby. I actually really enjoy the dub, they had a lot of fun with it and it adds to the style a little.
DOUBLE EDIT: I'll come back to this thread later when I've had time to think about some more underrated gems at the moment my mind is a bit hazy.