Check and see what focal lengths you tend to shoot at. If you have Lightroom/Aperture that should be pretty easy to do in the filters. If you tend to shoot all the way at the end of your zoom, look more for a 70-200, or other telephoto (or go prime, 135 f/2 is nice!), or if you shoot mostly at the wide end, look at something there.
I definitely recommend renting something you're interested in, and I'd look seriously at a prime (fixed focal length) unless you're much more likely to use a zoom.
If nothing else, get the EF-50mm f/1.8 (aka nifty fifty). It's cheap, around $100, and it's a great intro to prime lenses, and on a crop it's ~85mm which is a classic portraiture length.
Other lenses I'd recommend (I have or have tried):
EF-85mm f/1.8
EF-135L f/2 (love this lens!)
EF-70-200L f/2.8 IS USM II (quite expensive, but rented it and it's on my list)
EF-24L f/1.4 II (again, expensive, but loved this when I rented it)
EF-35L f/1.4 (rented this, can't decided between this and the 24, but I'm waiting for v2 to come out)