I prefer the original one; Fullmetal Alchemist.
Because not only does it have better animation and voices, but they really worked on developing the characters and their stories well. The remake of it was really fast, and if I hadn't of read the manga before hand, I would have no idea what was going on.
I know the first series went off the original topic of the manga, but I really didn't mind because they are both really good sides of the FMA story,