Considering the vastly changing situations a landscape photographer is presented with (or looks up actually), I'd start by making a list of features you want from a lens.
Obviously you will want several focal lengths, so I'll assume your landscape kit covers at least wide/ultra-wide (between 16mm and 24mm widest) and moderate telephoto (between 135mm and 300mm or longer). I'm not picking ONE lens between wide, telephoto or anything in between for versatility, but because I think that if you're serious about this stuff, you won't leave the house with less than 3 lenses anyway.
My list would look something like this;
For a wide angle lens:
- Sharp when stopped down.
- Good contrast.
- As little CA as possible.
- Little distortion, or distortion that can easily be fixed (so avoid moustache like the samyang 14mm).
- Very flare resistant.
- Able to use filters, preferably the same size between other lenses (77mm is a nice average).
- Uneven number of aperture blades for highlights in night photography is a nice bonus.
For a telephoto the same criteria count, but some more than others. CA and flare mostly but a stable tripod mount and relatively lightweight are important as well. Aperture blades is less important but a nice bokeh can be.
For the OP; weathersealing might be a very important feature, but for me it's not so much. I rarely go out in dreadful weather, and if I do, I protect with plastic bag contraptions ^^
I really enjoy the Canon 17-40 + Sigma 50mm macro + Tamron 70-300 VC as a landscape kit. Very basic, cheap and lightweight, but it does what I want it to and never really let me down.
The 17-40 is often trashed for not being sharp but I like it when stopped down and on the modest resolution of my 5Dc it's very sharp.
The Sigma 50mm goes all the way down to 1:1 when I find nice details, and it offers a nice middle ground between a fast 50 and a full blown macro lens. It's also ridiculously contrasty (don't know how, it just is) which is really nice to squeeze out details in early morning foggy shots.
For landscape use, the Tamron 70-300VC offers 95% of a 70-200 F/4 IS or a 70-300L for 25-33% of the price, and I rarely use it below f/8 for landscape use.
I'd really like a 24mm tilt shift as well, but it's a heavy hit on my budget, so I'm waiting to see what the samyang 24mm will bring when it comes out.