I don't know the first album very well, but so far I would have to say Here We Go Again. Haha, I don't even know for sure that's the name! Oh well, but when I first heard it I really liked it, but apparently it's not one of the 人気 ones on the album.
On Riot!, I think the most underrated song is Fences. It's just an awesome song, and it's never talked about much.
On Brand New Eyes, I think the most underrated song is probably Playing God. In my opinion, it has excellent lyrics, and it's really catchy. My お気に入り is Turn it Off, but it's becoming もっと見る popular.