Honestly, I think they all have their advantages: cairo dock reminds me of compiz - it's very customizable and it has lots of special effects; docky is more simplistic, but still very nice, and awn is in between - it has effects, its pretty customizable, but it isn't overkill (not to say cairo dock is). You could compare cairo dock to KDE, AWN to Xfce, and Docky to LXDE. My personal favorite is AWN, but I'm using docky right now because AWN isn't in the Testing repo.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid