I would start out with box (buxus) and use Miracle Gro moisture control potting soil which is very rich and moisture retentive. If the box don't thrive you could go with euonymus which are more drought tolerant. However. euonymus tend to have greedy roots so they may not be the best neighbors to the other flowers in the box.

Changing out and freshening soil in window boxes is a must. And use organic fertilizer because chemicals will eventually turn the soil into salt.