UU is my favourite tier by a landslide so I love all dem bad Pokemon. My ideal meta exists as everything is valid to use in its own tier. Abolishing the whole thing where something is at the top of its tier list and/or is suspect, and then there's other valid stuff to use in the tier that you might want to use because it's still -good- but they also might be kind of meh at times and you wish you had something else instead.
I would say most things that are Ice type. I seldom ever make any builds for ice types because most of them are really bad, except for Cloyster and Weavile. Mamoswine fell from grace a bit on the current tier list apparently. People can say the same thing about bug being underpowered too but at least the megas introduced for the bugs are actually solid. Mega Pinsir and Heracross are amazing (Mega Beedrill is kind of lackluster I guess, but adaptability is god.), while Mega Abomasnow and Mega Glalie are a bit lacking. Most people are just happy to stick Ice Beam on random things that can learn it for the coverage rather than using something with STAB damage. I frequently have Water types with Ice Beam.
Other than that more specific examples of stuff that could be good but falls on its face are things like Rapidash, which has its niche as a physical fire type which you see I'd say I little less than special fires - but then Rapidash falls on its face against your Blazikens, Infernapes, Talonflames and gosh, even Flareon now is better. Rapidash has the build set up for a fast physical sweeper / revenger but it has no moves to support iself. Recommended build is Flare Blitz/Wild Charge/Drill Run/Wil-O-wisp. So you have 2 recoils and a status inflictor. Not what I'd ideally want to run for a standard sweeping set. You can kill yourself very fast with recoil (even moreso if you have life orb) and Wil-O-Wisp I prefer to run on bulky Pokemon in most circumstances.
I am also on Team Mega Farfetch'd. Let it Ch'ding everything in all its glory. It would be beautiful. So, so beautiful.