Till now (I'm currently on minus 33) the levels that I had problems with were:
18 (musical thing: I solved it mostly guessing)
30 (I tried almost everything, but on a wrong pic)
52 (it wasn't complicate enough, I complicated it more!)
61 (not because of the password "hacking", but because of a very strange logic of mine that told me I don't need to enter the mail archives)
70 (couldn't understand how to form a word from what I had)
74 (took me a lot to figure out the way)
80 (searching for chicks was a nightmare)
-21 (first elephant thing: I rearranged the code to look nicer
)
-24 (the first step was the hardest, but only because I made it so)
-29 (damn sequence!)
-32 (I was close to invent a new cryptography system
)
The list is still open
<edit> Added to the list: -34, beta, gamma (unfair from my pov), delta, theta and lambda. </edit>