"Best" is tricky.
In my experience, RebornROM is the best because it is hands down the most functional ROM I've used on my Optimus S. I like HydroRom more than Reborn, and Cold As Ice even more than HydroRom, but they were much buggier for me. Under HydroRom I simply could not get GPS to lock, and the phone would reset itself seemingly at random. Under CAI, my 3G data was abysmally slow, to the point of non-functioning.
It sounds like HydroRom behaves for you, which is great. You also might consider CAI. RebornROM will probably feel like a step backward for you.