I think it could be Bourussia Dortmund. with Gotze, kagawa, lewandowski, barrios and bendnar. Its an excellent team. All they need is 2 fullbacks and maybe a keeper and a brilliant team. They also have Subotic and Hummels which is really good. Who agrees?
I guess it depends on your definition of "underrated". Consecutive titles in real life means they're not entirely flying under the radar But they're a side you can definitely go places with, they have that massive stadium capacity, so there's always cash coming in as long as results are good. If you want to play them, a big challenge is hanging onto the top talents long enough that the club builds enough reputation they stop wanting to leave for that dream job at Barca, Bayern, Man City, so on. That takes a few years. I've had that problem with Bundesliga clubs that aren't Bayern (that is, all of them have the problem).
I think Napoli are another one, some astonishing attacking potential in the side if you can just complete the picture, again while keeping the players at the Cavani/Hamsik level from leaving first.