Fullmetal Alchemist: Brotherhood: I appreciate what both Fullmetal Alchemist, the 2003 series, and Fullmetal Alchemist: Brotherhood brought to the table in their own ways but I personally prefer Fullmetal Alchemist: Brotherhood for being more faithful to the manga as written by Hiromu Arakawa. It's just a fantastic series with a really fun story, great characters, good humor, good emotional weight, and a nice soundtrack. The 2003 series definitely did a better job with the bits that it had adapted from the manga, so that's unfortunate in Brotherhood's case, but I still prefer Brotherhood.
Yu Yu Hakusho: One of my earliest exposures to shounen anime and to this day a series that still holds up. The cast, especially the main four, are all pretty fun and iconic. The Dark Tournament and Chapter Black sagas are both excellent and the villains of those arcs are quite memorable; I especially adore Shinobu Sensui, the villain of the Chapter Black arc, who really resonated with me when I was watching the series and has been pretty influential to me in some or my own creative endeavors. It's a shame that the art took a hit toward the end of the series during the last bit of Chapter Black and at points during the Three Kings arc since things were obviously winding down, but it's still enjoyable enough. It's also somewhat of a shame that the anime adaptation didn't get to show the final case in Spirit World, though we did get that OVA a year or so ago that showed us that and it's probably for the best that way.
It also had, for the most part, a pretty great dub and one that I prefer since it was the way I engaged with the series first. I'd still recommend it to people to check out. I still need to get around to watching Hunter x Hunter though, which was written by the same author (Yoshihiro Togashi), since I've heard that it's quite good.
One Piece: This one is a bit looser in the standings than the other two for a few reasons and could likely be unseated by something else. There's way too much filler that runs overlong and that I have no interest in and skip. Some things even in the main story take absolutely forever (hi Dressrosa). There is a disgusting amount of episodes and still no end in sight, which makes it difficult for people to want to get into. Some issues and artistic decisions are handled...disappointingly. Despite all that, what it does well it does very well. It's a grand adventure with a fun cast and it will pull at your heartstrings at points. Oh, I can't stand to watch the dub though, the subs work way better for me and the original Japanese fits some characters like Scratchmen Apoo and Admiral Kizaru way better.
Other honorable mentions would be Azumanga Daioh, Cowboy Bebop, Dragon Ball/Z/Kai/Super, JoJo's Bizarre Adventure, Ouran High School Host Club, Paranoia Agent, Psycho-Pass, Samurai Champloo, and Trigun off the top of my head. I'm probably forgetting some stuff, but hey. Some other good stuff I have passing familiarity with but need more exposure to would be Detective Conan and the various Lupin the III series; I saw what aired on [adult swim] years ago but need to see more of both. I especially hear good things about Lupin the III: The Woman Called Fujiko Mine.
There are some highly regarded things that I still need to get around to watching in full that might become favorites and/or shake up my favorites, such as Attack on Titan, basically anything Gundam (never seen any of the various series but some friends highly regard some of them), the previously mentioned Hunter x Hunter, My Hero Academia, Neon Genesis Evangelion (I've at least seen some bits of it, particularly the last episode and the movie End of Evangelion but not every episode), Outlaw Star, Puella Magi Madoka Magica, Serial Experiments Lain, and numerous others. A good portion of what I've seen was aired on the original Toonami blocks years ago and early [adult swim], though others like JoJo's came to my attention through social circles in recent years.
