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I am looking to by my first prime lens and I am undecided whether to go with a 50mm or 85mm. I am unsure which would be more versitile. The question others would ask first is "What do I want to do with it?" I don't know. Any suggestions or preferences?
Get a zoom first. Shoot a lot with it. You'll figure which FL is most appropriate for your photography in this way.
There is no generic "best focal length" for a first prime, even aside from not knowing whether you shoot cropped sensor of full frame.
Then, unless you really need something very specialized, at the point that at which you figure out that you need some specific prime focal length, start out with one of the fine, inexpensive Canon non-L primes - among them the 85mm f/1.8, the 50mm f/1.4, the 35mm f/2, and a few others.