What is the best resource for finding an apartment in Denver?

I've found that the HotPads iPad app is one of the most convenient. If you want to go with a management company, Boutique Apartments and Wheelhouse Apartments are very professional to work with and they are very particular about the quality of their properties. http://www.boutiqueapartments.com and http://www.wheelhouseapts.com
If you're looking for a house (and they have some apartment too), you might check out Real Property Management of Colorado at http://www.realpropertymanagementcolorado.com

Trulia usually works pretty well. I think my friend used it to find his current place in Villa Park, called the Avalon: https://www.rentattheavalon.com/

Other than that, I've personally had good luck with Zillow. Thought it was mostly for buying homes but turns out they have lots of rental options there too.