Bengal Tigers and Black Rhinos for me. Something about their design always impresses me.
For tigers I just put one of each gender in an exhibit and ignore it till I realize their exhibit is empty again
For the rhinos it's a little different. I always put 1.3 rhinos in a vast termporary exhibit in the back, while a build up the rest of my zoo. Eventually it gets to the point I have 50+ rhinos. I would crate the ones I want to keep and release the rest to the wild. I would then build my African Safari section with giraffes, elephants, lions, etc. The rhinos are the ultimate mixing species in the game, as they get along great with every savanna species. I would put a rhino in every exhibit, complimenting cheetahs, zebras, antelope, ostriches, etc. very nicely