Hi. I don't listen to beethoven concertos much (not that there not good, there excellent, I just havent bought the recordings yet) so i'll leave them out of my list.
The list, in my own humble opinion
Difficulty Greatness (popularity??

)
Chopin Chopin
Saint-saens (very close with chopin) Gershwin
Schumann Saint-saens
Gershwin (although extremely difficult Schumann
if you dont get jazz *obviously)
If your deciding, try to pick the style that suits you the most. If you love chopin, go for chopin, its not OUTRAGEOUSLY difficult, but will require much time spent. (its 20 min long). But if you can play the gershwin rhapsody well, go for it. Its much easier technically, and only challenging if you havent played any jazz, but still its not THAT tough. Saint saens is a well respected concerto, but i think the chopin would beat it if played equally well IMO. Have fun deciding, theres probably no wrong choice.
SAM