Whilst not a Paignton regular I do get down there near annually so I'll have a blast at answering you. Personally I always feel that whilst Paignton is a nice looking, reasonably well laid out zoo with some good exhibits (I love the baboon rock) the species selection, for it's size, is a little "vanilla". That said it does have a number of oddities and rare (in zoos) species to look out for.
The real star for me is the Echidna, a monotreme -it's a mammal that lays eggs!! I know everyone knows that, but I still find that wonderous. There's very few in captivity outside of Australia and, as far as I know, it's the only one in the UK. Best time to catch it is 4pm to closing (that's been learnt over years of experience and a lot of wasted journies up the hill to it's enclosure).
Other rarities include Allen's Swamp Monkey, Mueller's Gibbon, Hartmann's Mountain Zebra and the Nine-banded Armadillo (in the nocturnal house).
Given you're Essex based, further intersting species for you might be Collared Peccaries, Cusimanses, and the Grey Kangaroos which, if I remember coorrectly aren't common in zoos near you. The crocodilian collection is also nice (but I'm sure that was on your short-list anyway).