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.
For me, it is a toss between Gardian and Mutalisk because Mutalisk turn-into Gardians. I chose Mutalisk, because it's best in destroying the enemy's buildings and ground-troops and air-force HaHaHaHaHa!
The time it takes to even make a BattleCruiser is way to late to match the time it takes me to make Mutalisks and annihilate your base!!!! HaHaHaHaHa!
I chose the tank... because, it's the most used unit and it's probably the unit that is the most powerful. What other unit can you put 6 of behind a couple of bunkers and make it nearly impossible for the enemy to reach you? Battlecruisers can be locked down easily. So can tanks, but not as easily, since a tank will hit a ghost before it even locks it down... most of the time (unless ghost cloaks). Mutas IMO are the least powerful units. If all my opponents when mutas, i'd be undefeated, cause they are rediculously easy to beat.
There is no such thing as a perfect defense. No matter how many tanks and bunkers u build there is allways going to be a way to brake it down. If u lock urself in the enemy will expand all over the map and get enough money to make guardians, queens (broodling), BCs with yamato, defilers (dark swarm), corsairs (disruption web) or carriers.
I DONT CARE WHAT ANYONE SAYS DARK ARCHONS RULE i mean u can mindcontrol a drone like that and you can be zerg and protoss you know how big of a advantage that is... i mean how many ppl here over look that advantage DAILY!
Mutas dont splash. Splash damage affects all the units in an area. For example, siege tanks can kill 5-6 marines in one shot, but mutas can only attack at most 3 units at once, no matter how many enemy units are at the hit location.