I've tried Arctic Silver 5, Ceramique 2, Arctic Cooling MX-2, and OCZ Freeze.
I definitely like MX-2 the best. It spreads easily, cleans up without too much effort, has better temps than AS5, and you can get big and gigantic tubes of it for relatively low prices. I picked up a 65 gram tube of it for $30 USD, and a 30g tube for $20 USD, the 65g tube lasted about two years, and I've had the 30g tube for over a year now and am just now getting low.
Hate Ceramique 2, when I got it the stuff was already dried up in the tube. I bought the Ceramique 2 and MX-2 at the same time, MX-2 still spreads easily, can't get the Ceramique out of the tube hardly at all now though.
AS5 is really messy to clean up, and the electrical conductivity/capacitance is too much of a worry for me since I've seen it kill boards and CPU's from spillage. There are better pastes out there nowadays anyway. This stuff was developed in what 2004/2005?
Didn't like the OCZ Freeze, it dried up in the tube with the cap on really quickly.