It depends of course on your budget.
Here are my picks for the best hotels/resorts in several price ranges. I've been visiting Placencia for 12 or 13 years and have written a half dozen books on Belize.
Top End:
Turtle Inn
Inn at Robert's Grove
Middle:
Kitty's
Mariposa
Luba Hati (high middle)
Low Middle:
Barnacle Bill's
Budget:
Tradewinds
Lydia's
Manatee Inn
--Lan Sluder
www.belizefirst.com