Hm...well, I like anime with maturity, seriousness, and action, but only a few of the anime that I currently "like" fit that description at all. I tend to pick "safe" lighthearted animes (Girls Und Panzer, Squid Girl), because I'd hate to be exposed to a full season of angst and grossness with little to no payoff at the end (hello, Utena...).
But some of my efforts did pay off:
-KARAS (So, so, so obscure, but very good. I never thought I'd actually like something so dark~)
-Black Rock Shooter (I'm a little iffy on this one. It's a good watch for sure- well-paced, great fight scenes- but I don't really understand it that well)
-Noir (Another one I don't fully understand...lots of symbolism. But the plot construction is just so focused and mature, it's gratifying to watch. Plus, guns! :D)
-Senki Zesshou Symphogear (This one is kind of a cheat, since it's popular in Japan...and in retrospect, I don't think the writing is all THAT great, and it does have its fair share of fanservice...but not NEARLY as much as most anime starring a team of heroic high-school girls. The action is thrilling, the music is great, and the plots are actually rather serious; I almost wish they'd take a little more time out to do innocent filler episodes...)
-Princess Principal (A new steampunk spy-thriller that I'm glad I took a chance on~. It just got a dub last month that's remarkably well-done. Sure, the voice acting itself isn't always top-notch, but the choices they made with the accents of the international cast show care and attention to detail that's pretty rare.)