I just purchased Canon 6D. I need advice on what is the best lens to buy for portrait photography...or what kind of lens I am lacking?
These are the lenses that I already have:
Canon EF 50mm 1:1.8 II
Canon Zoom EF 28-80mm 1:3.5-5.6 V USM
Canon Zoom EF 75-300mm 1:4-5.6 III
any advise would be great! Thanks!!
Lots of lenses make good portrait lenses (assuming by that you mean subject isolation + background blur), though which are most recommendable depends in part on what sort of portraits you're going to make and under what conditions. If you'll be fairly close, you'll want a shorter focal length (but probably not less than c. 70mm) and faster aperture. If you're going to be farther away, 200mm could be fine. Classic portrait primes are fast 85 and 135mm lenses. I'm especially partial to Canon's 100L and 135L and Sigma's 85mm 1.4, but either the much less expensive non-L Canon 85 or 100 might be a safer place to start, depending on your budget. Then again, you already have lenses which include those focal lengths. Do you want/need shallower focus? If nothing else, you can use the lenses you have to get a sense of which focal length you would prefer for portraits if you decide to go with a prime.
As for your second question - what am I lacking? - that's really up to you. Have you ever wished you had a lens that went wider or longer? Are you unhappy with the qualify of photos you take with the lenses you have? You would likely see an improvement (both in the resulting photos and the pleasure of simply using the lenses qua bits of machinery) if you replaced your zooms with, say, a 17-40L and 70-200/300L (it doesn't matter if you lack something between 40 and 70). Or with a 24-105L and 100-400L. Or you could go wild and get the 24-70 2.8 II and 70-200 2.8 II. (And if you like the idea of making your camera as small as possible, you might consider the wonderful 40mm pancake lens.) If you're lucky enough to live somewhere with a good selection of lenses to rent - e.g. in the U.S. and thus will access to lensrentals - you could always experiment and see what you're (not) missing.