If you're looking for 5 phones for testing purposes, getting the 5 best Android phones probably isn't your best option, you want options that cover the largest possible amount of Android's install base, which almost certainly won't be the 5 best phones.
I haven't done any significant Android development, so can't give real good answers, but I suspect you'd want to get a Galaxy Nexus and a Nexus S for sure, at least one device that runs Froyo and one device that runs Gingerbread, one device with the lowest possible hardware specs you expect users to be using your app with, with as many different screen resolutions as reasonable. Probably worth getting at least one tablet too, probably a Motorola Xoom running Honeycomb depending on how well you plan on supporting Honeycomb.
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