Levels 6-12 for me. I also like characters that still have to worry about many confrontations and that I don't have to create numerous god like opponents. They seem to fit easiest / best into the game and mood I create.
I have liked to run 1-5 up to now. but I am beginning to like 6-10. My group has only on set of PCs that have reached 11-15. These are old PCs that we modified for 3E when it first came out. These PCs are going into semi-retirement while we start with a new group of 4th lvl PCs.