It's interesting how the public are now interested in Meerkats, when not many years ago, you had to be an 'animal person' to know what one was. What I take from this, is that any animal can be made into a star attraction, with the right PR build-up. Meerkats are badically a jumped-up ferret (alright, not strictly taxonomically correct, but you get the idea. Lion King and clever insurance advertisement later, everybody wants to see them. Admittedly, sitting up like little people, helps.
However, I would suggest that, if you tell the public 'Come and see the Whateveranimal, they're amazing!' And tell them the ways in which the thing is wonderful, you will create a demand, and everyone will want to see it, and by extension, conserve it and its habitat.
My own impression of what the average zoo visitor wants to see is:
Meerkats
Penguins
Really big snakes
'Monkeys' -- you mean there's more than one sort?
Gorillas -- a lot of them are quite happy to assume Sulawesi Macaques are baby Gorillas
Lions
Tigers -- a surprising number of people don't really know the difference
Elephants
Rhinos
Giraffes
And that's about it.
However, if the right publicity is done on Owston's Civet, Kaiser Newts or Grosbeak Starlings, they'll want to see those.
As to what twenty species I would have, it's a bit difficult, as I've probably got about two hundred favourite species of bird, never mind anything else.
However, let's try:
Demoiselle Crane -- arguably the most elegantly proportioned of all birds
Red-breasted Goose -- conservation message, very beautiful
Lady Amherst's Pheasant -- it has everything: colour, shape, big enough to see
Laysan Teal -- fun little duck with endearing personality, good conservation message, interesting evolutionary history
Nene -- conservation success story
Dodo -- empty enclosure with signage and simulated habitat.Example of a species that was actually kept in Europe, and allowed to go extinct because nobody really understood the issues.
Passenger Pigeon -- another empty enclosure, with signage explaining how the species went extinct, despite being bred in good numbers in UK, captive stocks were allowed to die out.
Shama -- song, colour, shape, urgent conservation message with current 'mining' of songbirds in South-east Asia
Sumatran Laughingthrush -- important to build up and maintain captive populations, also does what the Shama does
Pekin Robin -- much as the Shama really
Ferruginous Pygmy Owl -- the public like owls, I like this species because it's an amazing little thing like a carnivorous budgie
Edwards' Pheasant -- possibly extinct in the wild, needs all the holders it can get
European Oystercatcher -- because I really like them
Livingstone's Fruit Bat -- needs more holders, conservation message, wonderful animal
Rodriguez Fruit Bat -- much the same really, not as impressive as Livvies, but massive teddy bear cuteness factor
Blackbuck -- one of the most elegant and prettiest hoofstock around, conservation message
Vine Snake -- just a really attractive species
Pied Tamarin -- needs holders, conservation message
Fossa -- fascinating thing that evolved to do the cat role in Madagascar
Clouded Leopard -- to demonstrate similarities in arboreal lifestyle to Fossa, graphic about convergent evolution
Left out lots of species, so hard to choose.