Generally I have seen people use the Blue Snowball and Blue Yetti but I would not call them the best...
In my opinion the best USB Microphone currently on the market is the ATR-2100 and AT-2005. These mics have become very popular in the podcasting world and also allow you to expand and use it via XLR.
I agree with Andrew. The ATR-2100 is a really good microphone. I always get complements from mine as to the quality of the audio. I also like that it can do both XLR and USB so that in the future if I want to get an audio desk I can and not have to worry about throwing out a good quality microphone. Plus it does even have a headphone jack too so you can monitor your levels right from the microphone. Also, it can be normally purchased for around $40 on Amazon so it is quite affordable.
I do not get why someone would get any of the Blue branded microphones over the ATR-2100 or AT-2005 models as they are not as flexible and do not sound as good. Plus the build quality is really good too.
Here's an audio sample I made on the fly a few weeks ago using the AT2005USB and Voxformer's vocal channel strip VST(Plug-in). The sound is quite comparable to the more expensive dynamic mics out there IMHO:
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