I have only ever been to San Diego Safari Park (When it was SDWAP) and Wildlife Safari in Oregon and Bear Country, U.S.A. in the Black Hills of South Dakota. Based on my limited personal perspective, SDSP is the better of the three with its even bigger spaces and mixes of species as well as much better walk-through village. That said, I did enjoy Wildlife Safari, too. Having white rhinos in the same space as your car is a thrilling experience, as is having a giraffe lower its head toward your open window. And having dozens of black bears surround you in Bear Country was fascinating to say the least.
SDSP also happens to be my favorite zoo anywhere, too, more so than Bronx (my second favorite) or the SDZ.
Highland Wildlife Park looks amazing from the pictures I have seen here, and while it doesn't look as great as SDSP, I've always wanted to go to Safari West in California's Wine Country after my favorite individual Oaklan Zoo giraffe moved there, and the aviary there looks great.