i haveta tell ya...
It is definatly the BC, in my opinion. also, there are othe strong units, but a fleet of these can wipe out, Hydras (without darkswarm) zerglings, mutas, devours and definatly guardians, dragon (but not without taking heavy damage),and all ground units that cant attack air, extremely fast.
Many ppl may argue the point they are "expensive" i just have to say, that far into the game money doesn't even matter. Amazingly, they can take out goliaths fully upgraded, if they are in a good situation, ill post more about that later also they take awhile, but so do carriers and they seem to be popular. and yamato cannon can kill turrets and cannons in a single shot. it takes about 6-7 to take out a bc wit scourges, and only like 5 yamatos to take out a cc or nexus. and they wipe out valks, corsairs and wraiths, scouts can do good damage against them though. anyways even if u dont have yamato, you could fly into the middle of maybe 20 cannons with 12 bc and yet, they will still beat them, trust me i know.