I'd echo the Xanadu choice- been there last year and couldn't get in this year, dang! Depending on when you plan to visit, rates can be as low as $160 for a loft. They're a bit of a hike from town (but walk-able all the same)-- we believe that makes the place a quiet retreat. Their beach is beautiful and very well kept. Terrific pool, great service from owners and hosts, who will arrange all excursions and taxis etc for you quickly and efficiently. Good snorkeling off their lengthy dock. Free use of bicycles during the day, and the all-important BACK UP gen-set for when the power conks. http://www.xanaduresort-belize.com/
We stayed in a 2BR garden view and were very pleased with the quiet, the efficient AC, the well-appointed kitchen, the roomy showers and great water pressure, the fun hammock hung on our little patio, the landscaping, the friendly staff and the closeness to restaurants and SP town. We were treated like family and thoroughly enjoyed our stay.