In order of Price High to Low (if I were just going to get one lens)
Canon 28-70 f2.8L (about $1200)
Canon 28-135 f3.5-5.6 IS (about $400)
(This would be the one resonably priced lens that I would recommend)
Canon 50/1.8 (about $80)
The Tamron lens mentioned may be a good lens, but I personally don't trust Tamron lenses (primarily from bad focus and noticeable distortion). The price seems to be about the same as the Canon 28-135, but the Canon has Image Stabilization. On the other hand the Tamron mentioned could substitute for my first two lenses the 28-135 and the 75-300, except for the IS.
If you went with one of my first two choices then the second lens I would recommend is to follow with:
Wide: Go with 16-35L or 17-35
Telephoto: Go with 70-200L IS or 100-400L IS or 75-300 IS (note the trend here)
And no matter what I highly recommend getting at least one prime that gives you at least f1.8 if not f1.4
If you can part with the money get the 'universal lens' and then pick up the 50/1.8 or 50/1.4 for portrait/snapshot work.
Just my opinions,