A Rivoli's is easy to ID as it is a large hummingbird, probably the largest you'll see in Arizona at 4.5-5.5 in (11-14 cm) long. Despite their size, they are not aggressive to other hummingbirds at feeders. This female is beautiful, but not as beautiful as the male, which has a magnificent purple crown when the light catches it right. I am still waiting to get a good photograph of a male! 
Until recently this bird was called a Magnificent Hummingbird, a more appropriate name than Rivoli's, I think.