I prefer ones with decent structures or an easy level of competition - the Australian-only ones on Full Tilt, for example, were great for a while: only 700 people were registering, and of those about 400 just sat out. So they were free blinds city. They've gone a bit downhill lately though, now they're averaging about 1000 registrations and less are sitting out.
Always found the CC freerolls to be decent too, though the time difference with daylight savings is stopping me from playing them at the moment.
But the regular ones with thousands of entrants and tiny prize pools? Ain't real enthused about them