Card Captor Sakura... :)
That show was great. I remember watching it as a kid and loved it.
I have watched some stuff that was pretty bad but stuck with it or ended up being embarrassing to watch but they are not really guilty pleasures to me. One series does come to mind though.
Aria the Animation
I love this series. It is not exactly the fastest paced show ever and nothing really happens... at all. I ended up watching every season and never regretted it for a second. The reason it is a guilty pleasure for me is the show does not really fit my personality and I will never mention I watched it to anyone I actually know.