Full Metal Alchemist: Brotherhood
Hardly an unknown anime, this is often cited as the best manga/anime ever made. A bold statement, but one that has a lot of backing. Hiromu Arakawa's manga was completed and released in 27 volumes, but I admit that I've only seen the anime. From what I hear, if you haven't seen the anime you should watch 'Brotherhood' as opposed to the stand-alone title. The non-Brotherhood show is the same as the manga for a while, but the plot diverges because the episodes were produced faster than the manga could come out. The anime was later re-done as 'Full Metal Alchemist: Brotherhood', following the original more closely.
First thing to note is that I got dragged into this anime kicking and screaming. I had started too many anime's with young teen heroes and rarely made it to the third episode. With FMA, however, the show throws you into a very rich and mysterious world that lives and breathes. The world of FMA has it's own history, and politics as well as the introduction of alchemy. Arakawa treats alchemy as it's own science and lays out a very consistent set of rules throughout the anime. All of this transports you deep into the interesting and believable world.
The show walks the fine line between staying smart and complex without becoming confusing or pretentious. I credit this to one of the show's strongest aspects, the pacing. The entire series is tightly plotted with a story simple enough to have mass appeal, but complex enough that it keeps you wanting to watch the next one... and the one after that... and the one after that. With most other long-standing anime's, the writer ends up spending half the anime introducing game pieces on the board then using the last half to play with them. There isn't anything wrong with this method, but what FMA does differently is introduce new characters, philosophies, politics, and plot turns all the way to the end. This gave a constant sense of discovery and adventure.
The ever-growing cast could definitely have backfired, but Arakawa does an amazing job of designing and fleshing out a wide range of characters. FMA somehow manages to bring out each of the personalities without ever having to slog down the plot. Another novel and refreshing approach Arakawa takes is how she under-powers the main characters. FMA doesn't succumb to feeding a viewer's power fantasy, but instead humanizes the two brother protagonists by having them be two pawns (albeit important ones) in a bigger story. Arakawa uses this to her advantage as she masterfully lets the viewer explore the plot along with the protagonists without too much undue complexity.
Full Metal Alchemist: Brotherhood ranks as one of the best anime's I've ever seen, and I am tempted to grab the manga box set even after watching the full series. If you really want to see what an anime can do or want to binge on a great story with some friends, this is the one.