I've used the Tunze DOC 9010, which worked ok, but the build quality seems cheesy given the cost. It was very stable and wasn't over sensative, builds a head pretty quickly.
I eventually replaced it with a Bubble King Mini 160 which is a beast for the small footprint, 7.5"x7.5". If you have never seen a Bubble King skimmer in person, you are missing out. I dare you too look at one first hand and not buy it. You see where the money goes.
You can also check out the Elos skimmers. I have not used one, but they have a ridiculously small footprint and apparently are great performers as they are paired with the high end Elos tanks.