Master of Orion/Planets to colonize

Deciding which planets and which order to colonize them in is more of an art than a science.

There is often no clear right or wrong answer. Unlike in the beginning of the game where there are often courses of action that are almost always correct, everything beyond your first colony landing is essentially up in the air.

The different types of stars, habitability, and resources have all been described in other sections, so general considerations are all that is really needed here.

Generally speaking, the biggest concern is to grow empire wide production at the greatest possible rate. This allows for planetary defenses and ships to be constructed at the greatest possible rate and allows for the greatest level of empire wide safety while resources are being devoted to research.

It is important to note here that research is controlled by factories, not only by population. If you want to get the maximum research rate from a planet you need to have the maximum possible factories on the planet.

Note: This is different from Orion 2, where you can forego building anything related to production and just put people on research and have them be maximally capable right away (assuming the research buildings are already constructed on the planet that are available).

The goal, then is to max population and factories ASAP and then dump everything in research spending, taking away from that as necessary to build ships and missile bases. The game AI will do this automatically by itself. It automatically spends everything possible on factories and when spending is reduced for any reason from any slider it automatically goes to research by default.