If a very serious Canon photographer is asked what 3-4 classic lenses he/she would buy if starting over again the answer would be THIS one in the wide-angle to moderate telephoto zoom lens:

http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_24_70mm_f_2_8l_ii_usm

It has a huge aperture (2.8) for outstanding low-light conditions, is sharp as a tack and renders incredible and consistently obtained images that one can make a serious living on. Like most of the top Canon lenses, it proudly wears the little red ring around the barrel denoting it is a Canon "L" series lens . . . the best they make. They hold their value extremely well and most photographers who don't have this lens yet desire to own one "someday" (and wish they'd bought one to begin with).

Truly a classic, outstanding lens.

They will next pick up this equally as incredible lens for covering the 70-200mm range. It has few equals at any price . . . truly a rare zoom to equal the very best "prime" lens at any of its focal lengths:

http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_70_200mm_f_2_8l_usm

I know quite a few experienced photographers who have those two . . . and most went through a lot of lesser lenses (to save money) before getting what they should have gotten in the first place!

The third classic Canon lens? This would depend on where your photographic interests evolved . . . whether super wide angle, precision macro or super telephoto! I won't go into those right now but wanted to mention the 70-200 2.8 "L" lens as a very worthy "next" lens.

When folks show up with these two in their bag I know right away that they are real serious photographers. From wedding pros to sports to nature, you name it . . . these two can produce images that truly "wow" people on a consistent basis!

Hope this helps!