Go for either 3, 4, 7, 15 or 20- these are the best known preludes.
But I also recommend 1, 6, 24, 16, 17, 18, 24 and Op.45 in C#m too.
#3 has a hard left hand, but the right hand is simply a piece of pie.
#4 has large chords on the left hand with an easy-peasy right hand
#7 is the easiest of them all.
#15 is the most popular and the longest, also belonging to the easier preludes
#20 is the funeral march- only 3 great staffs.
I've heard #s 1, 6, 24, 16, 17, 18, 24 and Op.45, but I havn't seen their scores yet, but these also sound as nice as the famous #s 3, 4, 7, 15, and 20.
#s 3, 4, 7, 15 and 20 are relaxing and belongs to the easier preludes except for #3, while 1, 6, 24, 16, 17, 18, 24 and Op.45 are quirky energetic, all belonging to the more difficult preludes. Which one do you choose?