I think gundam is a bit overrated, I really couldnt understand why people fuss all about it, maybe it's only me who dont enjoy mecha-themed anime other than macross though.
Dragon ball and naruto are obviously overrated. Dragon Ball manga is quite good, but the anime is just too much and the dragon ball GT is not even supervised by Toriyama. And about naruto... sorry to say, everytime I read the new volume of the manga, I lost in confusion about who is who and what is the main plot in the manga.
Jing, King of Bandit (Yuichi Kumakura) is very underrated. This is probably the best surrealistic manga you could find out there. Jing consists of two part, the first part spawned of 7 volumes which character design is similar with Toriyama's. The 2nd part, the Bottle, consists of (so far) 8 volumes and each volume is just very amazing, I praise the mangaka to think something very original and surrealistic. I have all the manga, and either the story is too hard or the manga translation at my country is bad. I feel the conversation in the manga is quite bizarre and not really flowing.
But the worst of all, people underrate Fujisaki Ryu's works (Hoshin Engi, Waq-Waq). I find it's a bit sad since his works are truly refreshing and out of main stream (He is hated by fans for killing a lot of characters in the manga). I enjoy every drop of Fujisaki's works, I hope he will be more popular in the future.
Other underrated mangas: anything done by Urasawa, probably because his genre mainly seinen. 20th Century Boys, Pluto, and Monster are even better than Da Vinci Code.
Jam it back in, in the dark.