Best landscape lens for Canon 50d

 Hi there. I assume you want something wider and better than the kit lens.
It sort of depends on your budget, but I would reccomend the Sigma 12-24 EX HSM lens. Equivalent to full-frame 19-38mm on your 50D, which is still pretty wide. Expensive at $1500 new, but you can often pick them up for much less (i got mine for $750 traded in). Covers full-frame, excellent build quality, fast focus, almost no distortion(!), and the best manual focus ring I have ever used. Pretty damn sharp, too.
Relatively big and heavy at ~600g, but a quality unit. Due to the huge front element, it is sometimes prone to flare, but is no worse than other ultra-wides.
Check out this site for someone who uses and loves his Sigma:
http://www.juzaphoto.com/eng/index.htm

Some other quality options are: Sigma 15-30 EX, Canon EF-S 10-22, Tokina 12-24 f4.

I have decided to buy only full-frame (non EF-S) lenses so that I can use them on all Canon bodies, but you may not care about this.

I have been using mine on a 20D (also tested on a 5D), and its great. When I trade up to a 50D very soon, I will be using it even more.

Currently using Sigma 12-24, Canon 28-135is and Canon 100-300.

The kit lens is actually pretty good optically, but the build and handling (manual focus etc) are crud.

Let me know if you want more info.

Again, it really depends on your budget.

Mike.