There were many good ones, but I can think of four that have an especially nice combination of presentation and spirit: 24 (wrong labels), 29 (crypto entrance test), 30 (sudoku), and 36 (the number e).
The only two I didn't like are 35 (blackjack) - unclean and tedious (on the other hand, the method I used to solve it was most probably incorrect, so my opinion may be partially unfair), and the first part of 37 (sql injection) - unrealistic implementation (did you get my mail, by the way?). A lesser problem is that some challenges are a bit spoiled by too revealing titles (3, 23, 24). I also have mixed feelings about 34 (zipped) - on the one hand, I am not particularly fond of such kind of problems, on the other, I was able to advance a few levels on electrica with the knowledge gained. :)