If you want to look at 'broken set of strategies' more than the 'brokenmons' themselves, look no further than rain dance teams. (I'm putting these broken in quotes because I don't quite believe them to be broken just yet, but people disagree on what's the definition of broken. Rain Dance teams have been a big force in DPP UU, and with BW RU shaping up like DPP UU Lite, I can't help but seeing the viability of Uxie/Kabutops/Ludicolo/etc type of straightforward Rain Dance team... and they've gone completely under the radar. Look at the usage chart!
To contribute to the Yanmega discussion, I don't believe Yanmega to be broken. The thing is, people always talk about Tinted Lens Mega for its power, but speed boost for the ease of revenge kill. Specs Tinted Lens Mega is incredibly easy to revenge if you're playing offense, and if you're playing defense, you should invest in a dedicated special wall anyways, let it be munchie, clefable, lickytung, audino or whatever. (lolaudino) We all know speed boost mega ain't so broken because when the argument for banning comes up, people somehow never talk about the speed boost variant--they all know that bug buzz+air slash isn't the BEST coverage ever, and there's too many counters.