There is no "best" Sauce., only what is best for you. That being said, a mild tomato sauce flavored with ground Cummin, Coriander, Ancho Chile Peppers, and maybe a touch of unsweetened chocolate will provide a great sauce.
But be bold. Try a sauce made from chunky pineapple, chicken or pork broth, onion, ginger, brown sugar, a a touch of 5-spice powder.
Both of these sauces work with both green peppers and rice. For that matter, a simple compound butter would go very well with this combination, as would a beef gravy that has been thickend with a cornstarch slurry.
But I don't htink I'd top it with hot-fudge.
Seeeeeeya; Goodweed of the North