Ugh, struggling here...
So, I'm looking at my list and being very indecisive.
The Last Airbender is always a safe option...
But then again, there are tons of others!
Choices choices -.-
How's Soul Eater btw?
Is Claymore worth it?
Jormungund seems to have a character like L in it.
Stein's gate?
Like I said: choosing your next anime is like a freaking harem ><
But to be honest, I'm still looking for than anime which will top FMA:B. Legend of Galactic heroes will probably do it, but I'm saving that for after my exams.
If you haven't watched Airbender, do so- it starts a bit moral-of-the-week in season one, but picks up into a very strong story arc and character development during season 2.
Soul Eater is solid- certainly entertaining, and good choreography. A bit cliche at the end, but hey.
Claymore is a fine manga, but was a painful anime (which went off in its own direction). Basically female shounen, sort of.
Jormungund was... meh to me. Like a less intense Black Lagoon (which, if you haven't watched, I recommend you do).
Steins Gate is trippy. It's slow and confusing at the start, enough that I almost quit after the first episode, but that's very much a deliberate lull and false security.
Just to throw a few others in your pot-
If you want some solid giant robot action, both halves of Gundam 00 were pretty good in different ways. Season 1 was a deconstruction of the 'giant robot wars For Peace' trope that a lot of mecha have, with more focus on politics/mystery and less on the action (which was good but short), but Season 2 was an extremely top-notch mecha action show that blended the Real Robot genre with the Super Robot genre even more than Season 1.
If you want something a bit shorter, Star Driver was both over the top mecha and very self-aware silly. Good fights, amusing teenage hormone antics without getting sappy or lacking in self-awareness.
But if you really enjoy character development, enough to forgive dated animation and the cliches of the children's-monsters genre... Digimon is actually a very good franchise, seasons 1 (Adventure) and 3 (Tamers) especially. It is, without apology, about kids growing up with monster companions being the catalyst. They start young and flawed (but generally well intentioned and likable), and end up wiser, more mature, and better people by the end of the adventure that starts small and gradually grows in scale as they do.
Really, really good storymaking behind it. Annoying characters are generally annoying on purpose, and grow out of it. Plus, the english adaptation was well done in many respects- not just a literal translation, but fitting in more appropriate wording and inserting a steady sense of humor into it.