It's difficult to really help you without more info.
What is your budget for a lens ?
What other lenses do you have ?
What kind of shots are you most interested in creating ?
Will you keep the 15-85 for the small end of the zoom range ?
How are you going to replace the big end of the zoom range ?
Do you have definite plans to buy a full-frame body very soon ?
A constant aperture lens is nice to have. The 17-40 f/4 L will be a big upgrade in that regard.
But you're losing half of your zoom range going from the 15-85 to the 17-40. The 17-40 may be
an upgrade in terms of IQ, but it is not a perfect lens either. However, colors were fine.
I found that it was too short, the f/4 was too limiting and it did not have IS.
I am much happier with the EF-S 17-55 f/2.8 IS. For my usage, it is much better than the 17-40,
on a crop body.
I've never used the Tokina 16-28 - I actually had never heard of it. As I see it, the focal range
is designed to be a wide zoom for full-frame cameras. As the Tokina 11-16, Canon EF-S 10-22,
etc. are for crop bodies.