Overall, I prefer CliffsNotes.

CliffsNotes- The AP test preps are excellent, but for the literature notes, the are quite hard to understand.
SparkNotes- The AP test preps aren't as good, except for the APUSH one, which is rated better than CliffsNotes. The literature notes are easy to understand.
Princeton Notes- Overall the AP test preps are quite good. They are comparable to CliffsNotes.
Oxford- I like the IB test preps, since many other famous note-brands don't have IB-oriented notes.
Barrons- I haven't really read their notes, but I have heard that they are quite good as well.

Feel free to add other notes that you would recommend.

I prefer SparkNotes, I think they provide more information..sort of. haha.

I prefer Sparknotes. I think they're more comprehensive.

I've always used sparknotes.
never heard of cliffsnotes before.

How about PinkMonkey? Or Bookrags?

I also use PinkMonkey as well as Sparknotes, in case there's something that PinkMonkey picked up that Sparknotes didn't and vice versa.

barron AP books are great. i think princeton review AP is too easy.

CliffsNotes!! i dont think i really use any other