Probably the easiest would be Prelim 7. There are no 20m canter circles, easy change of rein in trot etc. However, the easiest test to remember is not always the best - some horses get bored quickly and some find the tests with more transitions and changes of rein better as it keeps the horse interested and occupied!
my horse hates her schooling so will probally find it boring lol she is still a bit of a baby but its worth doing it cuz i want to event her eventually
I find that if the test has lots of loops and circles the better!!!! My horse is like a rubber band and if there is loads of straight lines he is bloody hard work, especialy when he spooks at the arena boards!!
If I were you I would read through some tests and find one with movements that you know your horse can do.
After all the other week I did P10 and what a mess I made of the canter across the diagonals (ie I was tanked off with across the school on the wrong leg, thanks to a spectacular change in the corner, and no mater what i did the boy was not for changing!! result = 3 for that movement!)
I then straight away did P18 and got consistant 7's as it has circles and a serpintine.
It's up to you to try & vary her schooling so she doesn't get bored. Dressage or flatwork is incredibly important, events are won & lost on that aspect. Many withdraw if they don't get a good dressage score as they won't be able to catch up.
Additionally when jumping you spend 90% of your time on the ground so your flatwork has to be good to put you in correctly to the fence. Stick with it , it makes an great difference to your performance if your horse is well schooled.