I think my favorite would be the contented purr I hear when I give them their veggies, or when they return to their pen after staying at the vet's (for boarding, when I am away for several days). Or maybe the mixture of complaining noises and contented purring I sometimes get when I pick one piggy about of the cage, which he doesn't like, but he knows he's about to spend time on the cat bed and get a carrot to eat, which he likes a lot.
*wheek*wheek*wheek*purrr*wheek*wheek*purrr*wheek*wheek*wheek*