I'm surprised no one's mentioned Haibane Renmei...by far the best anime I've ever seen. It was so beautiful. The art, tone, characters, and I could go on forever about how good the soundtrack is. It's sad that a lot of people snub it because there's no cute, big chested heroine, no gun fighting scenes, or anything like that...
The story takes time to develop, but once it gets there, it's really amazing.
I also love Michiko to Hatchin. Characters and story are unique interesting. I love the art style, too. The show is hilarious and keeps me on the edge of my seat, a very fun and quirky watch.
Kuuchuu Buranko: Haven't really watched a lot of it yet, but I can already tell that it's awesome.
Di Gi Charat: Okay, not the best anime...but it's one of my guilty pleasures. It's one of the first anime I got into when I was young, so it's special to me.
Aoi Bungaku: I watched the No Longer Human portion of it. The character is both pitiable and easily related to. I also liked the style of artwork they used, I think it suited the dark tones of the anime (rather than using, say, a style like CLAMP).
Sayonara Zetsubou Sensei: A friend introduced me to this. I thought it was going to be a stupid, fails-to-be-funny anime. Well, I was really impressed. I love the humour they use, the characters, everything.
The Not-So-Good:
Hyakko: I don't know why I started to watch this, considering I don't like the whole 'School life with cute girls doing quirky stuff' things, like Lucky Star or K-ON, but I guess I wanted somewthing light-hearted and fun. Didn't like it. The story never really got anywhere, the characters were boring and cliche (except for the girl who likes to eat, forgot her name), and the art/animation was really mediocre.
School Days: I actually saw the ending of School Days first, which got me interested in it. But watching it from the beginning is a really hard task. Boring, boring, boring. As it progresses, you want to facepalm. There characters act stupidly and deserve the consiquences of their actions.
Elfen Lied: Well, I just have a really weird love/hate relationship with the manga Elfen Lied. I felt that it had a lot of potential, but there weere too many things that weren't done right. The anime takes a step even further into the bad catagory. While I can say a few good things about the manga, there is nothing I liked about the anime at all (apart from the fact that I liked its opening song).
Higurashi no Naku Koro Ni: This was also recommended to me by a friend. I was really into the first two arcs (Rena's door scene thoroughly creeped me out), but after that it got boring and predictable. Higrashi, like Elfen Lied, tries too hard to be 'scary' and 'intense' by using mindless blood and violence; after you watch a few episodes, though, you get used to it and it fails to be shocking anymore. Once you get over that, there isn't much left: extremely annoying and stupid characters (main guy in particular), friendship speeches (we can do anything as long as we believe in each other!), and bleh art.
Persona: Trinity Soul: As a Persona fan, I was a bit disappointed by this. Seemed like an extremely beefed-up shonen version of the games I love. I found it really boring, couldn't make it through the first few episodes. Maybe I should try watching it again.
And of course, whatever's popular with the 13 year old boys now (Naruto, Bleach, Inu Yasha, etc).
