welcome to the board. Saying "the best burrito" is a lot like saying "the best sandwich", there's probably no such thing. Some days you like one kind of burrito over another as your tastes change, just like sandwiches or any other food. With that said, Waraguma's are big-really big. They're probably as good a value as aything you'll find on the island. It's a little stand on middle street, just north of Rock's store if I remember correctly, on the west side of the street. There's a little store next door, where I often buy some fruit and whatnot to round out the meal. Jeff's recommendation to eat on the beach is right on.
As I recall, Canibal's made a good burrito, but the last time I looked it appeared that they'd closed down. I can recommend the tacos at Celi's Deli. They're good and cheap. I would not be surprised if they make a good burrito, too. Estel's would be a good bet.
I don't know if I've seen sour cream on any mexican food outside the USA, but I suppose it's available at some of the resorts. The cheese in Belize is only fair. After living my life in the southwest, I've never gotten a taste for rice in a burrito. Arroz is a side dish, isn't it? In the end, burritos are just like sandwiches, you can put anything on them that you like.
I think I'll head up to the store and buy a hunk of beef. All this typing has gotten me hungry. It's time to make some burritos.