Most overrated - Junjou Romantica, Sekaiichi Hatsukoi.
Most underrated - Too many. Kizuna, Love Mode are the only ones that come to mind right now.
While I absolutely love Junjou Romantica and Sekaiichi Hatsukoi, I do have to agree with Mia, they are overrated. I would also add to the list Border and Super Lovers. (Even though you asked for yaoi I’m going to sneak this shounen-ai in: Komatta Toki ni wa Hoshi ni Kike!)
For underrated I’d have to go with New York New York. It doesn’t have bad ratings in any way, shape or form, but I feel that not a lot of people have read it. I don’t know if it has to do with the fact that it’s an older manga, or that the way it was posted in this website makes it harder to read.
For me, personally I think that the most overrated yaoi manga would be Finder. It is a good yaoi manga but honestly, for me its just meh~
As for underrated..... I would say Kimi ni Ageru and Koi no Mannaka. Both of them are so good and brilliant that its such a shame that people are giving them the attention...
Overrated: Yarichin
Underrated: Vassalord. I don't know how popular it was when the anime and manga were ongoing but I don't see anyone mentioning this.
What do you think are the most overrated and underrated yaoi??