Really depends on what your looking to do with your phone... AOKP, Cyanogen, Paranoid android... Then your going to want a kernel to back it up. I personally use Franco nightlies but thats just what I like, you'll have to experiment with dozens of them to find whats best. Obviously if you don't already have CWM or TWRP installed you'll want to do that and make your life so much easier. I haven't checked to see if Paranoid android has released an s3 specific Rom but im using version 3+ on my galaxy nexus with franco kernel r364 right now and its fantastic. Do a quick search for it on here, Loads of customization.
