Fullmetal Alchemist, the original. It hasn't moved on my list in over a decade. It has consistently stayed as number one. It's the anime that truly got me into anime, saved me from my despairing teenage years, and has my favorite anime character/husbando of all time in it (Alphonse).
I am aware that Brotherhood is "superior" since it follows the manga's original storyline, but it missed something the original series had. It misses the heart the original series had. Sure, it eventually got that feeling toward the end of the series, but Fullmetal Alchemist felt like a passion project from the beginning in the original series. Plus, call me cynical, but I actually like the first series' ending, as much as it didn't make sense.