Rachmaninoff preludes op.32

[the green piano man]

Hello i thought about learning the whole series. I am currently playing the op.32 no.10 prelude and it is truly amazing. I played some of rach pieces and I truly love him thats the reason I want to learn the serie. My one friend would play the op.23 on a concert thats the plan. My curosity is which ones are the most difficult technically and overall