Situated at either ends of the resort. The Fiesta Beach is situated close the the 5* Helena hotels and is right on the beach Both hotels are very close to bars and restaurants and would only take 15 mins to walk to the very centre. Some reviews here.
http://www.tripadvisor.co.uk/Hotel_Review-g499086-d667339-Reviews-Fenix_Hotel-Sunny_Beach_Burgas.html
http://www.tripadvisor.com/Hotel_Review-g499086-d502947-Reviews-Fiesta_Beach_Hotel-Sunny_Beach_Burgas.html

For me I'd go for the Fenix

Toonman

Not sure is you have booked a hotel but if you are stuck try the Smolian which is a Balkan hotel
Dee