Debian Squeeze the most powerful (as of now), staying nearly 3 months and I am still using it in my netbook and desktop. Every app in linux is there in their repository and the rest is easily compiled or easily available. Uses very less amount of RAM and UI and app runs faster even in my netbook, not to mention how fast it feels in my desktop which is a core i7. Debian Testing may be the full stop for disto hoppers. I have already tried various kernels (2.6.30, 2.6.32), kernel compiled by myself, liquorix kernel, compiled firefox, used sidux repository kernels, mixed unstable and stable repos, pinned packages, tweaked settings, added multiple desktop environs, ran virtualbox, changed ext3 to ext4, changed ext4 to ext3, installed google chrome, tested all my python/java/gcc stuff and it still running, running without a sweat. Debian does not interfere with what I am doing and it helps me to shape it as I wish, and I don't see any other linux coming near its power as of now.
Ni sama ne bih bolje rekla.