For the animals then the lemur walkthrough enclosure at WMSP is probably pretty good. They have a massive area of mature woodland with some very tall trees to live in. The only problem is that you don't really get to see the lemurs, as aside from the ground dwelling ringtails they seem to spend most of their time up in the canopy.
Depends on your criteria but I love the capuchin monkey enclosures at Monkey World, particularly the one in the middle of the park. It's brilliant to see the ex-lab animals busying themselves with all the rich opportunities it affords - natural foraging opportunities (worm innards but not the skins seem to be a delicacy), high trees to climb, logs to investigate aside from any enrichment they may be given.
Best for viewers or for the monkeys? Barbary macaques enclousure at The Monkey Sanctuary, all the enclosures are excellent though.
I suppose just to clarify best for viewers and inhabitants alike - personally i do think that all these animals should have a chance to go off display at anytime they wish .
