Someone else started the thread about best zoos in the world, but personally I wouldn't consider traveling to the Americas just for visiting zoos. However, I would consider planning a short holiday just for visiting zoos, so I more interested in this list.
Some general observations on when to visit (at least in the Netherlands):
1) If planning in the weekend, go on a Saturday(morning). Every family in the Netherlands is buying groceries at that time, so no screeming children.
2) Don't go when it's very hot for Dutch standards. Even though most species originally came from tropical locations, they grew up in this climate, so you will only see animals hiding from the sun and doing absolutely nothing.
Some general observations on when to visit (at least in the Netherlands):
1) If planning in the weekend, go on a Saturday(morning). Every family in the Netherlands is buying groceries at that time, so no screeming children.
2) Don't go when it's very hot for Dutch standards. Even though most species originally came from tropical locations, they grew up in this climate, so you will only see animals hiding from the sun and doing absolutely nothing.