Definietly 9s. They don't tire me out (for the most part) theyre usually very fun IMO and in many cases theres a big difficulty leap from a 9 to a 10 chart
10s are my second favorite. Usually tiresome but I feel good passing one with a good grade.
11-12 I can only do on keyboard and I don't have as much fun with them. I like long streams without jumps, and the difference between a 10 and 11+ stepchart in SM can be jumps.
I can't even do 13 footers on a keyboard decently.