Fullmetal Alchemist does not have a complex timeline. Whether you watch FMA first or FMAB is also a personal preference. If you do not wish to spend time watching a non-canon and slightly shorter plot, watch FMA first. Regardless of what you watch first, both provide a unique and amazing experience.Which Fullmetal Alchemist should I watch first?
If you only want to hear the original story, you should watch the first 25 episode of FullMetal Alchemist first, then start the entire FullMetal Alchemist Brotherhood series. Do not skip the first 10 episodes of FMA Brotherhood either because there are some very slight changes that were made within those 10 episodes.