The best snow for skiing is usually Feb - early Mar, IMO. You have less issues with the snow getting slushy in the afternoons as you do in late Mar, early April.
Much better chance of Colorado's famous "champagne power" as well -- dry, fluffy powder which is the absolute best snow to ski on. Later spring snows can be big, but often they are a wetter snow and not as good as the dry snows.
After end of Feb, the crowds really increase. Unless you are skiing in the back bowls or other more advanced areas, you will be spending more time waiting in lift lines. But the weather is nicer, so it is a tradeoff.
Simply put, there is no "best ski week", just a lot of choices.
Kurt
Edited: I also want to add, if you are staying a week in Breckenridge, definitely check out some of the other great ski resorts in the area as well. You can drive to Keystone or Copper Mountain within 20-30 min from Breck. Keystone is usually included w/ your Breckenridge lift ticket, and has some fun night skiing (If you haven't tried it, you just have to experience it at least once). Arapahoe Basin is good for a bit more advanced skiiers and Vail (45 min) is definitely worth the drive for a day of skiing there.