If you plan on continue using APS-C cameras then you should consider EF-s lenses. I used to own a canon 10-22 and I really liked it better then my 17-40 /4 L on my 30D. And still, I liked it better as a superwide zoom on crop then I do with the 17-40 on my 5DII. But I had to upgrade camerabody and I don´t look back.
So, take a close look at 10-22, 17-55 /2.8 and 15-85. Non L? So what? Never mind that. The high quality ef-s lenses are great. Find out what focal lengths and f-stops you need, then take your pick.
For landscapes you probably step your lenses down to f/11 or f/16 or something and use a tripod so maximum apeture does not seem that important. Perhaps Samyang 14 /2.8 is interesting? Manual focus and manually setting apeture but for landscapes that should be easily manageable.
Another thing with ef-s lenses is they are often lighter, smaller and cheaper then EF lenses and that´s all good. Anyway the important thing is that there are great non-L lenses out there.
Take a look here http://www.bobatkins.com/photography/reviews/best_canon_eos_lenses.html
You will find that the ef-s 17-55 /2.8 is rated above the standard L zooms 24-70 and 24-105 even when price is no object. You will also find that only the canon 70-200 zooms and the sigma 300-800 are getting higher scores. The other lenses are primes and often L primes with crazy high pricetags.
Good luck and have fun building your system (and shooting with it!)