Tough question -- 42,706 variables...LOL.....

Examples:  in studio or outdoors, your preferred working distance from the
subject, off a tripod or not, flash or no flash, etc., etc......I shoot
primarily outdoors from a tripod with off camera fill flash (if needed) and
prefer the 70-200 2.8L (basically the same setup used for a lot of my animal
"portraits" -- the zoom allows for differences in setting up the shot and
ability to either defocus the background or have it sharper.  A buddy shoots
mostly in studio and prefers the 85mm 1.8L with back drops.....another
prefers the 28-70mm 2.8L in studio......in other words, your preferences.

And, as you probably know, you don't need the sharpest lens available if
you're just doing people.....most humans, including myself, look better a
little fuzzy.....hee, hee.

Mike

24-70 F2.8L USM

$What would you recommend as a portrait lens for the 20D?

  There are lots of focal lengths which are sometimes used for
portraiture, but the traditional range for 35mm film is generally
around 85-135mm, with shorter focal lengths being used in smaller
studios and/or for compositions including the upper body, and
longer focal lengths being used in larger studios and/or for
tighter head shots.  With the crop factor on the 20D, that's roughly
50-85.

  My 50/1.4 makes a lovely portrait lens on the 20D; for instance,
the third through sixth pictures at
http://www.stevedunn.ca/photos/friends_and_family/ were all taken
with this combination.

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