Watch Fullmetal Alchemist: Brotherhood if you only want one. It’s the more faithful adaptation of the manga and is widely recommended as the better single pick; if you have time for both, watch the 2003 Fullmetal Alchemist first and then Brotherhood.

Which one to start with

  • Choose Brotherhood if you want the fuller, more complete version of the story and the safest “best first watch” choice.
  • Choose the 2003 series if you want a darker, more original take that eventually goes in its own direction.
  • Watch both in order only if you know you want the comparison experience: original first, then Brotherhood.

Simple recommendation

  • One show only: Brotherhood.
  • Both shows: Fullmetal Alchemist (2003) → Fullmetal Alchemist: Brotherhood.
  • Skip mixing them episode by episode because they diverge enough that it gets confusing.

Quick takeaway

Brotherhood is the better default pick, while the 2003 anime is the interesting alternate version.

TL;DR

Brotherhood first for almost everyone; 2003 first only if you plan to watch both and want the full contrast.