For a full frame 5D MK II like yours, 85mm and 135mm are classis portrait focal lengths. 100mm is ok as well. As the focal length gets longer, it tends to flatter a face, while wider angles must be used with care to avoid having noses look too large.
Even 200 and 300mm lenses are used by for portraits, you just need to get way back.
My favorite is the 135mm L, but I have room to move back from the subject most of the time. Otherwise, 85mm is a good choice.
Exactly right! Portrait photographers, eons ago (right, Mt. Spokane?), determined that a mild telephoto (80mm-120mm) was just about ideal for photographing the human face. They offer just the right amount of compression to flatter the average face. Have great portraits been taken with a 50mm? Sure! How about a 200mm? Yes again! But you can never forget that you are dealing with human subjects; and they might not all be family and friends... To get a nicely composed head shot with a 50mm, you're really into someone's personal space. You can get a nice, tight shot with a 200mm; but you're so far back you're practically yelling at your subject to get them properly posed... I like the 85mm because it gives you some room to work, but still keeps you close enough to communicate... Don't forget to microadjust the AF if you like the wide-open, limited DoF look... And if you put a UV filter on for protection, don't buy cheap glass... Hope this helps!
5DIII, XSi, A2, 10s (x2), 20-35L, 24-70L II, 28-70L, 70-200L, 70-200L IS II, 135L, 40 Pancake, 50 Macro, 100 Macro, 135 Soft-Focus, Crappy EF-S Kit Lens, 430EX II, Metz 45 CL-4, Metz 60 CT-4 (x2), White Lightnings, and lots of other junk...