Hmm, by "underrated sweeper" I am going to assume you mean not OU? Because if it is OU, then it is certainly getting its usage.
So let's see, potential game breaking sweepers in UU or BL that perform very well in OU.
Staraptor and Kabutops would be my initial choices, and they have received their kudos so far.
The other obvious ones to me are Shaymin, Medicham and Azumarril.
Shaymin should have been OU long ago, as Seed Flare + Leech Seed + HP Fire makes Shaymin one of the most versatile Grass Pokemon in the entire game, so I'll skip that reasoning for now.
Medicham is an amazing Pokemon, however. Just for a basic comparison, Adamant Choice Band Heracross reaches 574 Attack and 269 Speed. Life Orb Jolly Medicham reaches 569 Attack and 284 Speed. This means Medicham reaches the same Attack as Heracross, higher Speed and no limit of Choice (I am using Adamant on Heracross specifically because the most common set is Adamant, and it requires Adamant for many of its common 2HKOes, if you decide to argue for Jolly, Medicham becomes 10% stronger for less Speed).
Granted, Heracross gets Close Combat, while Medicham only gets Hi Jump Kick, meaning Heracross actually hits 20% harder with its Fighting move, and with Megahorn it can take care of Cresselia. However, Medicham sacrifices power in its Fighting attack and the ability to beat Cresselia for the ability to OHKO Gyarados, Gliscor and Weezing (something Heracross cannot do), and of course, not be limited by Choice. I'm not saying Medicham is more useful than Heracross, I'm just saying it deserves to be used a lot more than it is.
Azumarril also deserves much more use. Forget about Belly Drum, and look at its impressive HP stat and decent base 80 defenses. I've been running a cool RestTalk set on a cool little team of mine (that btw, I am very proud of right now, you guys might see an RMT soon)
But of course, I am using more as a bulky physical attacker, not sweeper necessarily, so I'll desist on that end for now.