I'm just gonna throw out my two cents in response to Myzozoa's comment, I'm not sure if using Scizor as an example would really prove the point. Plus, 27% usage in OU is still completely ridiculous. But, the reason it isn't 45% is because there are just some teams that don't really need Scizor. Sure, it could be higher, but that doesn't make a Pokemon underrated. I just don't really think Scizor can be considered "underrated" because every knowledgeable OU player recognizes Scizor as serious business.
I think that for a Pokemon to be underrated if it has a niche that most players ignore, as explained by the definition in the OP. Scizor's niche in OU is one that everyone is aware of. But, the niche of, say, Togekiss, maybe not so much, so one could argue for that.
Yeah, I'm thinking that too. An underrated threat is something players tend to overlook or don't prepare for because they don't face it often. I don't use Scizor right now because it doesn't fit with my OU team, but at the same time, I have used Scizor before because I found its role to be very useful for my team. Also, on pretty much every single team I have ever made, I've thought "what Pokemon (plural, for that matter) can handle Scizor on this team." Not every team needs Scizor - if this was true, then Scizor would be on every team. No Pokemon is perfect, but there are some threats that while you are team-building, that just stick out so much that if you ignore it, you won't go very far. The same goes for stuff like Terrakion or Rotom-W.
Although he isn't an offensive Pokemon, Skarmory is a good example in a lot of Generations. In generation III, he was probably sitting at about 20-30% usage, which is around where Scizor is now, and even then, I believe he rarely topped the statistics. However, he was by absolutely no means underrated. People packed the useless Magneton JUST to get him out of the way. Stuff like Salamence at this point were using Fire Blast just to get rid of him (and Forretress) exclusively. Rapid Spinners were used commonly thanks to him and his spikes, and a lot of teams packed teams that didn't mind spikes. He didn't get GSC Snorlax usage, but he certainly helped shape the metagame. Similar stuff includes everything running Hidden Power Grass just due to Swampert, Scizor creating a rise in Heatran at the end of Gen IV, Blissey causing Special Attackers to struggle, etc. Similarly, Scizor's presence helps Heatran, makes some Tyranitar run the Steel-Resist berry, makes Reuniclus a bit uncommon (it can beat Tyranitar in Trick Room sometimes, but it has no hope against Scizor), is the core of the infamous Voltturn strategy, etc.
I see where your coming from in this scenario to whoever posted the original post about this, but everybody prepares for Scizor and a ton of people use it (or have used it at some point). Nobody really prepares for Quagsire anymore in OU, and people tend to ignore threats like Latias and Virizion, which is what makes them underrated. My opinion isn't correct by any means - it is just what I think.
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Onto the actual subject, I think that
Poliwrath is a little underrated. I don't think it needs extraordinairy uage or anything, but Circle throw is very useful to stop momentum for U-turn spammers (not Volt Switchers, though), it can absorb Scalds from some users thanks to Water Absorb, which prevents the burn from it too. A bulky set with Circle Throw with other moves like Waterfall, Scald, Ice Punch, Ice Beam, Rest, Sleep talk, Bulk Up, Toxic, etc. works pretty well. It can outspeed Skarmory and Circle Throw it before it gets phazed itself with little investment, and depending if it is specially defensive or physically defensive, it deals pretty well with Tyranitar, Politoed, Scizor, Ninetales, Heatran, Landorus, Volcarona, Terrakion, Infernape (Grass Knot hurts a bit though), which allows him to rack up entry hazard damage (and Circle Throw damage) once some threats are gone. A huge plus is his typing and ability, which gives him resistances to common moves like Fire Blast, Stone Edge, Bullet Punch, U-turn, Crunch, Ice Beam (Most of what any Tyranitar uses, Volcarona's STABs, and Scizor's 2 most common moves) and an immunity to the aforementioned water moves like Scald. While his attacking stats are pretty bad and he suffers 4MSS, he still has a very useful niche, and deals pretty well with several common Pokemon.