I would say that the Wild Animal Park would be the least crowded since it's further away from the other 2 places. I believe that when I went last summer, it was on a Sunday and the line for the Journey into Africa tour was pretty short. It's a nice way to see some animals w/out having to walk a million miles, but it'll cost a little extra though. I didn't stay there too long though bc my dad was sick, but when we went to see a show, it wasn't packed and we got a good seat.
Besides the WAP, I'd say that the zoo is the 2nd one to be the least crowded. If you were to do Sea World or the zoo on a Sunday, the only troubles that you might face is waiting in a long ride for a ride/tour or trying to find a good seat for a show. Other than that, there's a lot of exhibits that ppl just pass by and you can definately take your time w/ those.