EF 24-70mm f/2.8L...
NO ISEF 24-70mm f/2.8L II...
NO ISEF 24-70mm f/4L
IS... not "III" or "II" (there's only been one version of it to date)
All three are superb lenses.
The 24-70/4 is the newest (December 2012), smallest, least expensive and is incredibly close focusing (0.70X). It is the
only Canon 24-70mm with IS, uses a 9-curved-blade aperture, weighs 600 grams and uses 77mm filters.
The 24-70/2.8 II was introduced just a little earlier the same year (September, 2012) and many reviewers still call it "one of the very best of it's type". Some said it was like a "bag full of sharp primes". It uses a 9-curved-blade aperture and can focus to 0.21X. Canon revised the lens hood so it's not as huge as the one on the earlier model. This lens is more expensive, weighs 805 grams and uses bigger, more expensive 82mm filters.
The 24-70/2.8 original was intro'd in 2002, was one of the best of it's day and can focus to 0.29X. It uses an 8-curved-blade aperture, uses a rather big lens hood (which works well), weighs 950 grams, and uses 77mm filters.
All three have excellent image quality. Better than any of the third party options, especially when used on full frame cameras that don't crop away lenses' soft corners. Between the two f/2.8, I think the newer one gains a bit of sharpness at the wide end, compared to the earlier... especially when both are used wide open. But in all fairness, 24mm is very, very good even on the earlier lens and they are quite similar through the rest of the range. Wide open the f/4 lens has some very slight softness in the corners around 50mm and just a hint of it at 70mm, but otherwise pretty much matches the sharpness of the two f/2.8 lenses. Stopped down to f/5.6 or f/8, much of the differences disappear.. Of course, the f/4 lens cannot blur down backgrounds quite as much as the f/2.8 lenses.
See for yourself:
https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=787&Camera=453&Sample=0&FLI=4&API=0&LensComp=101&CameraComp=453&SampleComp=0&FLIComp=4&APIComp=0Finally, I'd note that the guys at Lensrentals.... who have experience with a large number of each of these lenses.... noted that the original 24-70/2.8 would get out of calibration and need occasional adjustment. Of course, their lenses spend a lot of heir lifespan bouncing around in the back of delivery trucks, going to and from renters. They also probably aren't as carefully treated by renters, as they'd be with their own lenses.