This depends on the time of year, and somewhat on the time of day as well...
During the winter months (just passed), 10/12/15/20/30m all would have been excellent choices. Now that we're entering the "summer doldrums" for HF propagation, your choices are bit more limited. I'd opt for 20 meters, probably, as the most all-around "reliable" band for daytime propagation that will provide contacts.
Don't worry about being "QRP," because half of all CW operations nowadays is QRP, and I think the number is still growing. I work CW mostly, on 40 through 10 meters inclusive, and above .030 (that is, 30 kHz up inside the lower band edge, and above) virtually ALL activity is QRP...regularly 100W, and often times a whole lot less. I think the "average" station on 30m is probably running 5W, and about half of all stations on the other bands are running in the 5-10W range.
So, you're not just a "little QRP" station on CW these days. It's becoming the norm. The work is all done by the antenna, anyway!
73 de Steve WB2WIK/6