NEVER NEVER NEVER NEVER EVER heat a ball joint or any steering or suspension component with a torch! There is a nylon bushing in the joint that melts really easy and you WILL RUIN the joint! Also never heat with a torch any components that you plan on reusing because you will anneal the metal and weaken it. I'm sure that the guys who claim that they do it all the time will protest me but I do this for a living and I have taught Steering and Suspension at a Trade school. I have seen it happen many times.
The best way to do it is with a ball joint separating tool that uses screw pressure to pop the joint from the knuckle. A pickle fork works too but you risk damage to the boot. Most of the time guys just take the castle nut loose and then hit the knuckle with a hammer right outside where the tapered shaft of the joint goes through the knuckle. Hit the knuckle not the arm or joint. Just be careful not to hit the threads with the hammer. Put an old castle nut on the threads to protect them. Use a good heavy hammer. 32-48oz ball peen or a mallet. A few good wacks should pop it loose.