Fullmetal Alchemist is often cited as one of the greatest anime of all time, but it's not always clear which series people are referencing. There are two versions of the anime - Fullmetal Alchemist, which came out in 2003, and Fullmetal Alchemist: Brotherhood, which was released in 2009.
There are some major differences between the two Fullmetal Alchemist anime. For starters, the 2009 series is far more faithful to the source manga, since the written version did not conclude until mid-2010. However, this gave the 2003 series a chance to take some interesting creative liberties, like turning Ed and Al's mother into a homunculus.
There's much debate over which series is better, and there's no objective answer, since both versions excel at different things. Even if you're a diehard fan who refuses to watch both shows, it's important to recognize the differences between the two anime, as each has qualities that merit celebration.