You don't need to watch each and every one to enjoy a compete FMA experience — in fact, I'd recommend against it. Should you watch Fullmetal Alchemist before Brotherhood? Image via Hulu. The Fullmetal Alchemist anime series and movies are not part of a shared continuity. Here is the correct order to watch the franchise.
If you watch FMA after Brotherhood, you aren't likely to enjoy it as much as if it was your first time since its more dated and less compact. Brotherhoods experience would become better by having watch FMA, as it builds the characters up more. If you aren't really feeling like watching 51 episodes to get to the newer version definitely skip them.. If you want the full experience, watch FMA before FMAB because of how the endings and how different the stories are. Because FMAB was considered the better adaptation, watching the 2003 version wouldn't feel as good after coming back from watching FMAB.