The best non-ideal worlds to look for are those with ideal temperature and pressure that just require a little more oxygen to move them from colony cost 2.0 to colony cost 0.0. These can be identified using the Colony Cost window (Ctrl-A). Changing temperature can take some time using greenhouse gases and anti-greenhouse gases although its worth looking out for those where you can raise the temperature a little and cause the ice-caps to melt, which raises the albedo, which in turn can have a major impact on temperature for worlds with a lot of water. For game purposes this happens at 245 kelvin, or -28C.
Gases to be removed can be identified by looking at the atmosphere notes on the F9 System View. This also shows the colony cost for each of the different factors affecting the planet. The overall colony cost used is the highest one. Pressure only becomes a real factor if you exceed the racial pressure tolerance, although it has a small effect on temperature.
Steve