What's the best focal length lens for landscape photography?

You don't supply enough information to accurately answer this question.

Are you shooting with a 35mm, 2.25 x 2.25, 4x5, 8x10? Are you shooting a digital or film camera?

In a digital SLR I like the 10.5mm .
I still use an old Hasselblad SWC which I love for landscapes.

I seldom break out the view camera anymore. I'm just too tired or too lazy.

Fixed 14mm or 20mm is cool. Also you can try Ultra wide angle zoom like 10-22mm.

18mm to 35mm lenses

More great photographs have been taken with a "normal" focal length lens than all the other focal lengths combined. In 35mm format a "normal" focal length is 50mm to 55mm since the angle of view approximates that of the human eye.

Wide-angle lenses are also good choices.

depends on the landscape and the image to be made,

generally people go for wides like 20-35mm or super wides like 12-20mm, i like 50mm, some landscape shots could be great made with a 400mm

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This means different things to different people. Some would want broad sweeping views and others would want to stand on a mountain top and zoom in on the next closest mountain.

See: http://www.flickr.com/photos/samfeinstei...