When I first explore a new system, I count the number of planets, asteroids, and stars, and give the system a score, as follows:
star = 40 points
huge planet or asteroid field= 5 points
large planet or asteroid field = 4 points
medium planet or asteroid field = 3 points
small planet or asteroid field = 2 points
tiny planet, moon, or asteroid field = 1 point
I total up the points and type the number in the system notes. I give priority to colonizing systems with the highest scores (assuming I have already researched high levels of colonization, stellar manipulation, and planetary utilization.)
(If you multiply the score by 6, you get the total number of facilities you can build in the system if you have advanced storage ability, convert all asteroids to planets, colonize everything, convert all atmospheres, and build sphere worlds around every star. Trinary systems of course have the highest score.)