I think that is completely subjective...
Rainbows would be the fewest in numbers and limited by spawning habitat. So by numbers, rainbows.
Growing up, I primarily fished stocked waters around pittsburgh, so the brookie was the most prized for me based on where they lived and the need to "get away" to find them. I still think they are my favorite because of where they live and how beautiful they are.
Wild browns are typically prized because of the size they can grow to... 20"+ wild browns aren't common place, but they aren't unheard of either. And there are certainly a good number of places to find them. There are many famous streams with great hatches where browns flourish. So you can get into the wild browns while still chasing fabled and prolific hatches.