With Nex 5 or 6...what fast lens would you...

If you want autofocus then this is where the NEX line lacks. But you can get an LA-EA2 adapter and use the Sony Alpha lenses, which includes a 70-200/2.8 and you'll have autofocus. A lens of this speed is going to be large even on the NEX so using an adapter for a little more size doesn't really hurt anything. If you can go manual focus then there are a ton of options.

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60 of my favorite shots from 2012