But I'll have to go with C.C. from code geass but of course, they're many more girls out there in the anime world that deserve to be praised for their valuable assets.
Fate Testarossa...For being both my favorite character and having an all around amazing butt...And her barrier jacket is comprised of wearing a thong. <3
I have many favorites butts and it's very hard to choose (which is why it probably took so I have many favorites butts and it's very hard to choose (which is why it probably took so long to pick a favorite). So I'll just list some of my favorites, and I'll warn you now that all links are to NSFW sites. And to warn you, I have a lot of girls to go through...
First off I'll say that I love Arima Senne's plump butt. She's my waifu though, so of course I do. However, she's not actually "in" any show or official manga so I don't know if she counts.
Second place, and thus "first" as far as media goes is Kan'u (Aisha) Unchou from Koihime Musou. I mean, just look at it <3, it's just begging to be poked. I may be biased as Kan'u has a great rack and I could stare at both ends.
After that comes another Tall Dark and Bishoujo Honor Blade, Saeko Busujima from Highschool of the Dead. Dat thong and apron combo, what can I say.
Speaking of black thongs, I do agree with Killuan that Fate Testar-ASS-a from the Nanoha series is very sexy. I do prefer her StrikerS or older forms, but that's just personal preference (and speaking of Nanoha, while she wouldn't "place" by herself, Nanoha Takamachi has a gorgeous butt too. Fate's is more well known though).
Unfortunately, I can't discuss Supergrunt's choice as I haven't seen Code Geass (though I do know that it's filled with enough beautiful butted girls to be called Code GeASS). And of course I know Zettai's love of his Mikan buns. Of course, because I know that I was less inclined to enjoy hers while watching (By all means, she's got a great butt, but you laid claim and I respect that). If I had to choose a To Love-Ru girl it would probably be Yui Kotegawa's shameless backside. However, TLR is more of a "bust" show for me, and compared to other Xebec shows with girls like Chizuru Minamoto's "delicious cake" (from Kanokon) or Hedyeh's sweet brown ass (a rarity, it seems; She's from Ladies versus Butlers!) it's not as impressive. In my eyes, at least, and TLR is still better than quite a few shows butt-wise.
Next up comes a "tie" in Yukie Mayuzumi and Momoyo Kawakami, both from Maji de Watashi ni koi Shinasai. The reason that they tie is that while Momoyo clearly beats Yukie in a side-by-side comparison, it's far more likely to see Yukie's butt as she seems to be the designated "Ass character" so she gets far more screentime (Momoyo has huge breasts so those get shown off. I am not complaining).
While I'm talking about butty black haired girls with big breasts, I might as well throw Shizuru Fujimura from Shinkon Gattai Godannar into the mix. Heck, her big, thong-covered butt even had problems entering a mecha at one point! She also has the distinction of probably being the character from the "oldest" anime on this list too.
Last "mature" girl for now is Horizon Ariadust from Kyoukai Senjou no Horizon. She might be mostly cyborg and a "weapon of sin", but it would be more of a sin to not count her amazing butt! I'd like to see the sun rise on her "horizons"!
And now for something completely different: Lolis! Yes, there are a few immature girls that I like the butts of (Nanoha and Fate have been mentioned, though mostly Fate). However, I would like to say that I do like Hinata Hakamade (the one on the right) from Ro-Kyu-Bu. There's also the girl that got me to realize that lolis are fine every once in a while: Shana from Shakugan no Shana. However, the best loli butt I've seen (or at least my personal favorite at this time) is Ririchiyo Shirakiin's lovely demon butt from Inu x Boku SS.
I suppose that I am-ASS-ed quite a few girls in these images, and these probably aren't even the only ones that I like as the list will probably continue to grow. However, if you eventually manage to look at all of them I'm sure that you'll see my tastes, sort of (man I tend to ramble a lot, don't I. Hopefully other members are willing to write and aren't intimidated by my discussion).
I really like your taste! Hinata is adorable and has a nice loli butt, and Mirim was easily my favorite of the Queen's Blade Rebellion girls! Though Izumi and Lyla were also cute and assy, I wonder if you like them.
Hi...I just joined your club and I love it already. My favorite butt is from the anime, Highschool of the Dead - Saeko Busujima. I know this girl was already posted but i couldn't resist.