I recently bought a Canon T2i with the 18-55mm f/3.5-5.6 IS II and 55-250mm f/4-5.6 IS II kit lenses. I primarily purchased this as an upgrade from a point and shoot so that I could get better travel pictures. We do a lot of camping and backpacking and after coming back from Yellowstone last year and being very disappointed with the image quality from our point and shoot and getting constantly frustrated at not being able to capture low light scenes, I decided that the extra weight of a DSLR on backpacking trips would be worth while. I started practicing and read a few books focusing on landscape photography and realized along the way that I really enjoy composing and taking pictures.

I am realizing already that the kit lenses are not going to cut it for me. While they have been a substantial upgrade from the P&S, I am still not satisfied with the image quality (especially from the 18-55mm, which is what I have been using primarily). I am mostly interested in landscapes and general travel photos. I'd like to be able to take good low-light photos as well.

I can't afford to upgrade everything now, so I'd like some advice on what will give me the biggest bang for my buck.

Here's what I've had my eye on:

Canon EF-S 15-85mm f/3.5-5.6 IS USM
Canon EF-S 17-55mm f/2.8 IS USM
Canon EF 50mm f/1.4 USM