My miner is of course best.
Ok, maybe I'm a little biased. But it is easy to use and you don't need to install it. If you have Radeon drivers and Java, just click a button to start it. Click my signature if you want to try it.
Phoenix was my favorite for a while before I made my own miner. But then it started to lock up if there was a network problem (like pool server restarting). I had to kill it and restart it to get it going again. Sometimes it was stuck all night, not mining. Maybe someone can confirm if this is fixed in the newer versions?
DiabloMiner seems like a solid product. Never had any problems with it.