Void Rays and Corruptors do well against them. Wouldn't know about Terran vs Terran units, since I don't play the race. :/
I would assume, not counting the yamato cannon or w/e. That two Void rays would kill one. Or like 4 hydra's. I could be wrong, but #'s are the key to taking them down as they fire slow but hard. One big unit isn't going to cut it cause of that cannon.
Actually Battlecruisers are much more powerful than they were in SC1. It'll take you 6 Hydras solo versus a solo Battlecruiser. The best hard counters for each race vs. Battlecruiser: Zerg - Corruptor Protoss - Carrier/Warp Ray Terran - Mass Vikings/Battlecruiser Do NOT use Thors against Battlecruisers. They only do 12 damage and they do it slowly. Thors just get rocked versus Battlecruisers. It takes like 4 Thors by themselves to take down a Battlecruiser.
void rays and corruptors are the direct counter. infestors could help (they can take air units now!). sentries also help pretty well, guardian shield does -2 damage, and the BC fires lots of low-damage shots.
for terran vs terran- mass vikings source: http://www.youtube.com/watch?v=-2zbvwbfltU source for why thors are bad against bc http://www.youtube.com/watch?v=a_US1ARYM4U&playnext_from=TL&videos=v5InsGaKnTo enjoy the replays!
INFESTORS!!!!! God I love mindcontroling the enemies BCs and then yamatoing his own tanks. It's AWESOME. Especially when you mindcontrol a enemy worker and start teching up to your own BCs >: ]
lol. thor does tiny damage, bonus to light. BC's have massive armor. thors end up doing only 3 damage per shot to BCs.
Go to the help section of the game. Click on BattleCruiser. Memorize/use common sense to understand what they are good for and what they are countered by. You should probably do this for every unit if you haven't already. Basic Homework wins many games.
As Protoss : Use mass stalkers with sentry guardian shield + High Templars, cast feedback because feedback is doing damage and the BC lose his yamato shot + Voidrays covered by mothership can destroy damaged BC-s easily, because some of them cant use yamato. Void rays cant help because vikings eat them.
Everyone's forgetting stalkers (get blink) do bonus dmg vs armored! They're quite cost effective. ________ Lamborghini Alar history
For zerg, corruptors. They have bonus damage against massive for a reason. Protoss, either mass stalkers, or high templar to feedback + void rays. Pure void rays don't work that well because they get one shotted by yamato cannon. Or you can get a combination of void rays + stalkers + high temps. Also sentry guardian shield helps.