Here in Boulder CO, Trulia seems best. But some of my clients have had good luck with Hotpads and Padmapper. And, of course, Zillow and Craigslist can be useful. In general, I would look at several sites, because each has some unique listings.
Trulia has a great deal of rental listings. There is also Craigslist, Radpad (mobile app) and homes.com. But be aware, there are so many scam listings out there on these sites, including Trulia. If you do find a listing that you're interested in, DO NOT WIRE them money. I would also suggest driving around the area you're interested in and look for the rental signs posted out front of homes. Good Luck with your search