Level 10 is my favorite level. The setup is really nice, letting you put lots of burning temples near the beginning and dens later on to hit anything that makes it past the front line. Also, the path loops around, so your experienced towers will be able to hit the enemy twice, rather than them running past and having only to deal with inexperienced towers in the back. Out of all the levels, ninjas pose the least threat in this one.
I like Avenue…easy, good for attaining high exp and using burning temples right at the entrance! I find Hydra a little longwinded as the path to the gems is soooooo long