I've hunted all the planets. They're all good, even excellent.
The challenge is finding a buyer for a given loot before I get bored and fly again. The less activity there is on a planet, the bigger a challenge this is. As much as I hate the fact, the only planet on which the auction has enough activity to be worth using is Calypso. Anywhere else, there is still lots of business to be done in each planet's trade channel (calytrade, arktrade, etc -- I think Toulan still uses its main planet chat #toulan for trading but not 100% sure). And there are always a few loots which sell well enough on each planet to usually be worth auctioning - for example on Cyrene the "hot" loots are the materials required by missions, and they almost always sell. What's hot differs by planet though, and most often it is necessary to hawk loots in trade channels for days.
On Cyrene, it's all about mission materials.
On Monria and Toulan, it's all about hides and other species-specific parts, and that trade is almost entirely on the street and in trade channels.
I still don't know what sells on Arkadia -- second biggest economy in the game, but slow as a slug -- and I'm badly out of touch with Rocktropia.