The original's soundtrack was just on a whole other level. This song in particular really stuck out to me.I didn't get to watch FMA until adulthood because the fans I knew in high school weirded me out with their fangirl squeeing, but I went in chronological-broadcast order and went with the 2003 series first since we picked up the first half at the Half-Price. I didn't really know what I was expecting because I didn't understand much outside of "short main character is an alchemist and his brother's a suit of armor because of it, also miniskirt armies and seven deadly sins personified". So imagine my utter surprise to find myself invested in a dark, tragic tale of atonement, wars, racism, and the consequences of playing God, and that's when I finally understood why it's so popular. The original series having a bittersweet anime-original ending was honestly perfect, but I was still interested in Brotherhood as well as to see how much Bones had improved as a studio since 2003. And Brotherhood was a bit jarring at first because it was more goofy in look and tone than before, which I don't mind, but it was still weird getting used to.
I do enjoy both series, though, Brotherhood expanding a bit more on its world was something I liked and watching Edward and Alphonse basically grow up fighting for their humanity was gripping, and not to mention Envy was more vicious than in the original anime. However, I really and truly like the Homunculi origins better in the original anime, especially Sloth and Wrath (Bradley being Pride originally was neat, though him actually being Wrath still works, too), and Lust had more character, too, and the music is also more hard-hitting in the original. There's good tracks still in Brotherhood, but 2003's soundtrack's more poignant, and I also find Dante more interesting than Father. While it's still nice that Hohenheim still had connections to the main villains from both versions, his connection to Dante I thought felt more personal. So at the end of the day, I have to go with 2003 as my favorite of the two, but only by the skin of its teeth because both series are excellent and that's a miracle.
But it still is a damn shame that only Brotherhood gets brought up nowadays, especially in the soundtrack department. The other day someone off-site said their favorite FMA opening was the third OP, and I went "Awwww yeeeeah 'Undo' is awesome :D!" and they replied with "Um... I'm talking about Brotherhood's :/". And it's an opening I don't recall at all (think I just didn't care for it and I listened to it multiple times) and it made me sad.