Sukhothai doesn't have much to offer for accommodations. There are just a few rather common hotels and a couple of newer resorts, some of which are quite nice. When selecting a hotel, you'll need to make a decision about which "Sukhothai" you want to stay in. There are, in effect, two Sukhothais: The "old" Sukhothai is the ancient city where all the historical sights are, and which was more or less abandoned in the 18th century. The "new" Sukhothai is the city that was built when the Bangkok kings had the area resettled. New Sukhothai is about 30 minutes drive away from the Old Sukhothai. There are hotels in both cities, as well as some relatively isolated resorts in the countryside. My preference is for a hotel that is near the old city. There are a few hotels that are within walking distance of the historical park. The Legendha is one place I can recommend, but the Tharaburi is also recommended as well as Le Charme. Use the search box on this page to search for the best rates from dozens of sites for all Sukhothai hotels from Hotels Combined.
There are also a number of guest houses around Sukhothai - both the new city and the old - which might be worth considering. See my friends over at Travelfish for an up-to-date list of places available.