What area of Brittany would be the best choice to stay for a week?
There are many areas of Brittany we could suggest – and we can give an answer that's more useful to you, if you can tell us a litle more about your trip. 1) Are you traveling by car? 2) In what month or time of year? 3) What are you most interested in seeing/doing?
If you're looking for a home base, then consider either St. Malo or Dinan. This section of Brittany is also very convenient the western region of Normandy. Beautiful costal drives. Check out VRBO for weekly home and apartment rentals.
Since Brittany is rather large, a "central place" to stay for a week would need to be near the geographical center. Unfortunately, that would not be coastal. So, I suggest you choose two locations. Dinan(medieval) or St. Malo (much livlier) would be one good choice. For the other location, perhaps Quimper or Pont Aven. Try to visit Carnac-the alignments are fascinating.
Once you start looking at driving times, it may be that you end up agreeing with the last post (Cynthia): that two home bases will make sense.
First I have to state my personal bias. I lived in Rennes for a year so I just love the Emerald Coast and the Coast of the Rose Granite - all along the Northern coast line. And I have friends living in the area that I like to visit. I took my family a few years back for a 10 day vacation for which we had two home bases: first one was on the eastern edge of Brittany, just outside Dol-de-Bretagne. We found a great b&b www.manoirdelagrandemettrie.com and explored Mt St Michel, St Malo, Cancale, Vitre and Fougeres from there. Our second base was much further west, still on the north coast close to Morlaix. We had a house on the beach (great if you have kids along) right next to one of the great prehistoric sites, the Cairn de Barnenez. W did day trips to see the small villages with their famous enclosed parishes (e.g. Saint-Thégonnec) and also a day trip to Carnac.