Ok, simple question, how do Protoss deal with battlecruisers? I've had some success with Stalkers, but I've lost against Terrans almost exclusively because of battlecruisers.

Pheonixs? Battlecruisers got nerfed so they should be even easier

Really? Every time I've used Phoenixes they've had their asses handed to them...

Get high templars to feedback battlecruisers with full energy, then focus fire with stalkers and a few phoenixes.  If they mix with marines, mix it up with feedback and psi-storm, with some sentry shields to reduce damage.

voids are your best bet, if you focus fire them they will take down the first battlecruiser eventually but once they are charged on the first kill they will eat through them

Phoenix SUCK against Battlecruisers. They do 4 damage per attack versus BCs thanks to their 3 default armor and 5x2 damage type.
 
Void Rays are your best bet with Stalker support. High Templars to weaken them and prevent them from using their Yamato cannon is very useful as well. Remember that if the Battlecruisers are in open ground and stacked, you should micro your stalkers under the BCs; otherwise they have a positioning advantage (Air units can stack to infinite, wheras ground units cannot).
 
If you think about it, a couple sentries with Guardian shield can help also by diminishing their damage a bit.

High Templar + Feedback should be really helpful.

A lot of Terrans are now complementing their 3rax push with a BC rush. A small MM force plus a BC with SCVs repairing it is very hard to scout. Normally, BCs can be taken out easily with VRs as they do very light air damage however if you scout 2 gas real early or something that spells danger, either try to tech as fast as possible to Void Rays or pump stalkers/sentries if you don't have time and hope you make it.

High Templars and Void Rays should be the way to go.

I agree with the above. Void rays take out bcs and High Templars work too if you are confident with your micro. 
 
However, I feel like there is a better strategy: Don't let them get to battlecruisers in the first place!  If you are seeing battlecrusiers at anywhere but the late late game, then you should be more aggressive, teching to battlecruisers is slow and takes a while.  
 
Or just tech to carriers =)

Get some templars to feedback that shit. Then, just about any anti-air should clean them up, be it a blob of stalkers, some void rays or even some carriers.

High templar are definitely helpful if you get Feedback out in time.
 
For late game when a mass of battlecruisers can be out, I'd say get a mothership and cast Vortex to split the force in two.  Quite annoying, I must say.

High Templars using feedback on the Cruisers is a great place to start.  Void Rays will do good against them as well assuming the BC doesn't just kill the void rays first, gotta have some stalkers or something else distracting the BC while you charge up the VRs.   
 
Really IMO the best defense is a good offense.  Just pressure the Terran player early and try and harass his minerals and gas enough so that he isn't able to afford the money/time to make BCs at all!

" Pheonixs? Battlecruisers got nerfed so they should be even easier "

Feedback and Void Rays.