What's the best mining option if you weren't worried about power costs?
At time of writing it still possible to run a GPU farm but at the speed of the increase in difficulty this will not last for long.

The second presumption with this is that you are not paying for the hardware, its very unlikely you would even  break even if you had to purchase hardware at this time. The only real option in a competitive market is to go down the ASIC route. 

Performance in a good quality ASIC can be 100 x that of a top end GPU. The price tag could realistically be about the same. There is zero comparison.