The Palms would be the most centrally located - easy walking distance to most restaurants, shops, tour operations. And right next door to one of the best beaches on the island.
Carnival, like what you are probably thinking of, with elaborate costume parades and mas bands, is celebrated in mid Sept in Belize. Great to come for that! Not so much for the Easter time carnival which is a paint splattering, egg throwing fest that can get quite rowdy.