I'll second Punto Azul in Maraflores
If you are at The Sheraton then go here too
http://www.tripadvisor.com/Restauran...ma_Region.html Love this place, ignore the menu because they either won't have it or suggest you don't eat it. Just ask them what they have and let them bring it. Choritos a la chalaca have always been very good there.
Thanks to the snobbishness of Gastón Acurio, buffets are sneered at, but I think this one is great
http://www.tripadvisor.com/Restauran...ma_Region.html and they will make you up a fresh ceviche right before your very eyes.
Happy eating