I'd concur with everyone else and go for Amazon World.
I visited them all last year and, as ZG says, they all have their positives and negatives.
Seaview, to me, was the least interesting (nothing too exciting species-wise) but it was nice to see three species of flamingos and it's great if you like feeding animals (wallabies and waterfowl).
Amazon World is, by far, the most interesting species-wise but be warned that some of the animals prove a little elusive and you'll have to do a few circuits or come back later in the day to see anywhere near all the animals (good luck with the Paca, I tried at different times on two separate days to no avail).
Owl & Monkey Sanctuary is so beautifully done and it seems every aspect has been well thought-through, it has a real charm and, personally, I'd rank it second highest on the island.
Isle of Wight Zoo is OK, reasonably well done and a nice selection of lemurs and big cats (including some interesting colour variations). If you're really into big cats this might be a first choice but, for me, Amazon World would always be the place I'd go if I could only visit one.
Note, I did a thread on my visit last year which may inform a little more and/or whet your appetite.
Enjoy your trip.