Volcanic Inferno
Advantages/Disadvantages: -70% power needed for units, -15% population, starts with Fusion Technology already researched.
Recommended for: Just about any player. Volcanic used to be the best planet type in the game, then the advantage was lowered, and then increased again until it became what it is now. It is great later on, when all other planet types need to use about 10% more of their land for powerplants.

Oceanic
Advantages/Disadvantages: +20% defensive power, +10% research required.
Recommended for: Newbies and people which don’t want to get landgrabbed. Would you randomly grab someone who has 20% extra defense early on? I know for sure that I never would… this planet type is great for people who start late and don't want to get beat up after leaving newbie mode, and 10% extra research is almost nothing, only a few extra scientists.

Mystical Lands
Advantages/Disadvantages: -30% attack time, -10% offensive power, starts with S.C.O.U.T.E.R. already researched and Fusion technology.
Recommended for: Active players, or people who want to take big risks and leave themselves open for a short while. It used to be a great planet type, but isn't any more, because of the increased attack time in the game and lowered advantage. It was my personal favourite planet type for about 15 games, but I don't recommend anyone to play as it any more, since most of the other planet types are better.

Mountainous
Advantages/Disadvantages: +15% income, -20% power production, starts with Laser Dragoons already researched.
Recommended for: It might be hard to not run out of power if you’re not used to Star Kingdoms, therefore, I don’t recommend this planet type to newbies. However, for everyone else, it is still a good planet type. It is especially suitable to play powerless with (see advanced strategies).

Desert Wasteland
Advantages/Disadvantages: +17.5% offensive strength, -5% population, starts with Dragoons already researched, and Frequency Decryption Center always at 25%.
Recommended for: People who want to make big attacks. Not suitable if you prefer to make small, safe hits. The FDC bonus is a very good advantage if you use to get robbed or probed a lot.

Multiple Terrain
Advantages/Disadvantages: Starts with Laser Dragoons and Dragoons already researched.
Recommended for: Beginners. No disadvantage might be good if you are not used to Star Kingdoms at all, but there are other planet types which have far better advantages than disadvantages. Laser Dragoons and Dragoons might be good early on, but a few weeks into the game, almost everyone else will have that researched as well. Then this planet type will be quite useless.

Forest and Wilderness
Advantages/Disadvantages: +15% attack time, +25% population
Recommended for: Everyone! Witch the changes made in the last games, it definately became the best planet type available. 25% extra population is a huge advantage for only one or two hours longer return time.

Terra Form
Advantages/Disadvantages: +75% research efficiency, -2% income.
Recommended for: If you find research important, this should be your planet type of choice. Good later on.