I have a couple different window box containers. One is filled with ornamental grasses and is consistently beautiful throughout the year since we don't get too much very cold weather. The other planters are annuals which I change from time-to-time throughout he growing season. Pansies are the old standby, but also nasturtiums and petunias work well for me.
The one sure way to kill a container garden though is to forget to water it during any warm dry weather. I'm lost my fair share of beautiful plants when I went out of town for a weekend and forgot to ask someone to splash them with water!
Chris