Well theoretically they get harder the higher the no is but it depends on your horse really. Mine needs to be kept kept on her toes and listening so she doesn't spook so sometimes I find the 'harder' tests better. The best is to have a read of them all and see which has bits in it that you like the most. If all else fails do 7 as I think its really easy (sorry haven't done it for ages so can't remember).
the only one I have ridden is Prelim 14 and found it quite straightforward and an inviting test to remember. It was my one and so far only attempt at dressage (we ended up coming 2nd) and as long as you ride the test fairly accurately you should be ok. A few people did have callers at the venue I went to but I learnt mine (mainly because I was only doing the one test)
I have only done a few prelim tests but I would say prelim 7 is the easiest. I messed up with p14 as you have to show 2-5 steps of walk in a couple of places and I forgot! P18 includes a serpentine which requires quite a bit of accuracy. And p10 involves a couple of turns down the centre line that I'm cr*p at as I always turn too late!
Good luck!