Jolteon is so Underrated, and he is a beast, he did most things in my old team, specially if you run LO>Choice specs.
Zapdos is far too underrated, I always have problems with him, but I hate to use it.
Breeloom, exactly, he is very good, but im far too lazy to use it.
Kingdra. He is very good, still its pretty hard to use it outside of RD teams, since, he needs either a tun of set up(at least) or a choice item. Still,the best about him is his type combo, only 1 weakness, that used to be not seen much, but now, latias appeared, and salamence is still around.
Shaymin. He is BL, but still, anybody likes shaymin, but the shaymin I'm using atm, breaks through teams, still the others aren't that good...
Electivire. He is barely used, and nly when people got at least 2 weakneses to electric, he gains alot of speed when coming on T-bolts and stuff, and EQ/Thunderpunch can finish most things.
Another underated OU is tentacruel. Even if he can be a spinner, a sweeper, a toxic spiker, special wall, etc, nobodylikes him, he's like a specially defensive forretress.
And togekiss...Para-flinch hax with togekiss is pretty fun xD and is also great, its like a specially offensive jirachi.
Those are some underrated OUs