Initially, I was going to wait until I had seen the anime truer to the storyline of the manga and/or the movie sequel, to have a broader view of the whole concept to form a more well rounded opinion. But it'll may take a while before I've seen one or both, so I'll start with the first one I've watched and that is the anime Fullmetal Alchemist.
Before divulging further into the storyline, Fullmetal Alchemist debuted in October 2003 and is loosely based on the manga of the same name. Because the manga was still in continuance, the writer Hiromu Arakawa opted to go in a different direction from the graphic novel. The 2009 anime Fullmetal Alchemist: Brotherhood was in turn more faithful to the manga.
Because I haven't seen the latter fully yet, I'll start with the one that came out first and to tell you the truth, I wasn't fully invested from the start. I like my anime's to be funny with a hint of drama - case in point Naruto, One Punch Man and Dragonball - and Fullmetal was a tad bit too serious to my taste. Only later, after a few episodes, were the comedic punchlines introduced. Only then I could've enjoyed the story more.
Genre (Adventure, Dark fantasy and Steampunk) aside, the theme was rather original as I haven't seen many movies, series and not even many books touch upon this topic. The main thread in this story is the use of and the laws surrounding alchemy: transforming by trading. How far are you willing to go to find or transform a philosophers stone (a common occurrence when it comes to alchemy) and at what cost?
Furthermore, I found the adventures, highs and lows of and lessons learned by the main characters and the overall story arc well written. Every ending of an episode left me with questions and from one question came another. The storytellers definitely knew how keep me engaged. My only caveat with the story was the ending. It was definitely not the route I had thought the writer would go for and maybe that's a good thing?