Most of the accommodations in Center City West are in the Rittenhouse Square [1] area. It is a lovely area to stay in the heart of the downtown shopping, theater, and business district, and an easy walk to the Parkway museums and even Old City.
A popular and pricey part of town, it is dominated by high-end chain hotels, but a few mid-range hotels and bed-and-breakfasts can be found as well.
Links:
[1] http://www.moon.com/destinations/philadelphia/arts-and-leisure/recreation/parks-and-gardens/rittenhouse-square