Rach three is very difficult, but its very pianistic like you said. Prokofiev 2nd is very difficult and perhaps is overtaking rach 3 in difficulty. Brahms Second is one of my favorits,but the last movement is sort of a let down in the ending, it seems Brahms just rushed to an ending. Rachmaninov 2nd is the perfect concerto in my opinion. People are always interested in the difficulty of pieces. it doesnt really matter. Alkan has the most difficult music, but its mostly Crap. Tchaikovsky 2nd is also very difficult, but not the best concerto.