Anything that gets a "Same-Type Attack Bonus" with Flail and has low HP and good/decent attack, like Dunsparce, Dodrio (my favorite), and Eevee. I think there are a lot of other ones too that people just don't look at too much because of the popular OUs and Ubers :/
Random Pokemon I think are underrated (most of them I plan on using sometime. Though, I'm not 100% sure these are all underrated, but I haven't played against them in a very long time so I'm wondering why people aren't using them lol):
-Mismagius, good speed/S. def, pretty much can do what Gengar does, but looks sexier ^^ and that means something!
-Electrode, very good speed, possibly faster then the popular Weaviles, Jolteons, Gengars, and Alakazams, and can be a neat thing to Sash ^^ maybe it's not underrated, but I haven't seen any being played in awhile.
-Lopunny: OMG this thing is awesome! WHY DON'T PEOPLE PLAY THIS!?....not gonna give anything away lol.
-Ramphardos: even more OMG, DO YOU KNOW HOW STRONG THIS THING IS!? use Trick Room or Baton Pass into it with a Ninjask to eliminate it's slow speed and it can sweep a whole party with its 450+ ATK at LV. 100!
-Drifblim, I was thinking.....Baton Pass, Destiny Bond, Stockpile, and Shadow Ball. I remember I had one built like that and it was a pretty good Pokemon. I think it's worth a try.
I probably would say the most underrated would be Dodrio or somewhere along those lines (I haven't looked through my whole Guide Book yet lol) because it's Speed and STAB Flail can be very powerful...