Sword Art Online. It just recently got scanlated in manga but the book is on Baka_tsuki. Its more of a romance/adventure but the protagonist hides how strong he is so it kinda fits.

I haven't read Ares, might try it.
I've read most of the mangas in the Hidden Potential and Hidden Power categories, and those that I haven't read are those that I will read once they are completed, or those that I don't want to read, or those that aren't scanlated.
I'll try Birdy the Mighty (II).
I don't like the MMO part of Sword Art Online, otherwise it seems like an amazing series.
Any others?

There are times when you will miss what you never had. I wonder how you will find what you so desperately need.

black haze

Black Haze
Not much romance but potential for some.

edit : damn, unexpected123 said it before 😛

For sure Black Haze

Needs Attention: Nemureru Hoshi