Are you asking what are the best and cheapest restaurants in the San Diego Zoo, or simply around the San Diego Zoo?
Inside the zoo, probably the best restaurant (and one of the most expensive) is Albert's (in the Treehouse complex next to Gorilla Tropics).
For cheaper eats, check out the Canyon Cafe or the Raintree Grill.
Read all about the Zoo's dining options in the first link:
For restaurants near the zoo, here's my picks:
For best restaurant close to the zoo, check out the Prado restaurant in Balboa Park, next to the San Diego Museum of Art. It is a bit pricey, but nice. For good cheap eats close to the zoo, go across the Laurel Street Bridge (west) and check out some of the restaurants on 5th ave like Max's Deli or Jimmy Carter's Mexican. or go north up to University Ave and try Ichiban or Mama Testa (There's also a McDonald's up there). There's a lot of restaurants within a mile of the zoo. Use the second link and click through the options.