Gravitation. It was one of the first series I ever watched, and I now realise that once I got past the "omg boylove teehee" of it all there wasn't a whole lot going on. However, I adored it when I was thirteen and still get a little nostalgic listening to the songs. I'm considering rewatching it to see if the '5' I gave it is justifiable.
Other than that, Highschool of the Dead. I really, really liked that show, and if I rated for enjoyment only it would be a 9 or 10.
Code Geass is disliked by most? That's the first I've heard that. Unless you meant it had a tremendous amount of flaws or was horrendously bad, which then would make it a perfect fit. Though, I have to say. Even then, liking Code Geass wouldn't really be going as far as a guilty pleasure.
I'll add to my list from before:
Phi Brain. It has it's flaws, I agree. But I like it a lot. It's disliked by a lot as well. But when most of the anime you watch are either action-adventure, moe or hentai, a nice, refreshing air of puzzles isn't really that bad, considering how I hated puzzles before I watched Phi Brain.
Sword Art Online. This one has it's flaws. The execution of the anime in particular. It doesn't do justice to the novel, which is tremendous. I wouldn't say it's the best, but it does not deserve the hate it gets.
To Love-ru (and Motto To Love-ru). It's a guilty pleasure. There is little to no plot, the MC is a pussy, yet he gets the all the chicks. Yet I still love watching them. Dat cloth disintegrating bomb. Dem cloth transformations. Dem boobs.
k-on, a-channel and other shows that are about cute girls doing cute things that doesn't really have a plot. its even worse when guys like me like it. something that i cant tell, even to my anime watching friends.
boku wa tomodachi ga sukunai, denpa oona to seishun otoko and other cliché harem, slice of life anime.
Ultra Maniac - This is one of my favourite series, but it's your typical shoujo. So why did I like it so much? I liked it because it had a cute, enjoyable story that made me laugh and a happy ending. I also like how there was enough drama to keep it interesting, yet the drama was very light and didn't leave me feeling depressed.
I My Me! Strawberry Eggs - This show has an average rating of 7.03, so it obviously isn't very popular. But I thoroughly enjoyed this series. It was a bit controversial and I'll admit the art is a bit dated, but I thought it had a good story and the ending made me cry.
Busou Renkin - This is your typical shounen in a lot of ways, but for some reason I really liked this series. I was sad to see it end.
Probably Sword Art Online. I know it's a pretty flawed anime and I wouldn't call it my favorite anime or anything, but for some damn reason I enjoyed watching it...well...at times...I have my beefs with the ALO arc.
Sei Juushi Bismarck, but in its american/european form as Saber Rider and the Star Sherrifs. Grew up on it and still love everything about the show despite all the changes made for the western versions. Most of it comes from the really great german dub that made the show what it is and wipes the floor with every other dub...not to forget the awesome Theme Song by Dale Shacker that completed the nostalgic goodness of the show.
"The individual has always had to struggle to keep from being overwhelmed by the tribe. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself."
Sword Art Online - The first half is a guilty pleasure of mine. I'll agree with the haters for the ALO arc. It's shit.
Hoshizora e Kakaru Hashi - I'm not sure if enough people know about this one for it to be a guilty pleasure. The series is funny as hell and I liked the love story. I'm pretty envious of Kazuma though.
Rurouni Kenshin: Reflections - I run into a mixed crowd with this one. It seems like people either love this or hate it. Personally, I loved it. However, it seems like fans of the Rurouni Kenshin manga say I SHOULD hate it. Oh well, not going to read the manga and I still like it regardless.
Another - I don't get why it gets so much hate. I thought it was a fantastic series. Sure, there were some pretty BS things about it but the good FAR outweighed the bad.
Sankarea - I think if it had a 2nd season, it wouldn't be a guilty pleasure. It seemed like a lot of the hate came from the lack of closure the series had.