I've stored the a-S, AtsA, and NTb releases. That means for 1080p AtsA is best except if you want an English dub, in which case you need to get a-S.
For 720p, NTb is best.
I don't like doing cross-resolution comparisons since they're always a bit iffy: a lot depends on the upscaler you use and thus on your hardware. If you got a good GPU and CPU, NTb with LAV and madVR using NNedi3 with 64 Neurons (or more) should be the way to go.
For Fullmetal Alchemist: The Sacred Star of Milos, go with UTW unless you want the English dub.