I've used lensrentals.com. They had the best price for me, and the best insurance coverage, covering things like theft that many other companies would not.
After having done a few rentals, it occurred to me that for longer rentals, it would simply be better to buy the lenses and sell them after the trip. High end lenses tend to hold their value extremely well. Camera bodies don't hold value quite as well, but you may wish to compare the price of this option... it still might make sense. Plus it's easier to get insurance from a third party when you own it.