Taste is obviously the most important, then Disease resistance. the other factors would be important depending on particular needs.
If you have a lot of land, tree size may not matter much, and if your selling your fruit, having a productive tree, with attractive fruit would be important.
One thing not listed, is fruiting season, it would be nice to have a great mango that produces fruit in the winter for example.
So for personal use, 1) flavor 2) disease resistance 3) Size
For orchard use 1)Flavor 2)Productivity 3) Color and disease resistance.