Desert - It has a sizable population, with descent resource asteroid mix, and lots of room to build.

Asteroids, for sure.  Asteroids are inexpensive and easy to capture and a massive boon to early-game economies.  With enough asteroids to fuel your early war effort, you can take what you need in the long-run by force. 

Beyond that, desert and terran planets start with a wonderful 3 logistics slots, which is very useful.  Desert planets in particular can be upgraded to have up to 9 logistics slots, which makes them perfect for development moguls.  Ice/Volcanics are nice if they have low militia counts or high extractor counts, but when you see a planet with a dozen frigates defending it and only two extractors, you just have to shake your head.

Terran of course.

Well, that's not really a colonizable planet in the first place.

Pretty much nothing competes with a fully-loaded neutral, though.  Costs you nothing but your colony frigate's time and you get three extractors that operate at 110% efficiency.  Unless your opponent is Vasari, they're effectively untouchable, too.