I'd be wary of an ultra-wide like the Sigma 10-120 or Tokina 11-16. Yes, they're good lenses, but they do kind of back you into a corner composition-wise. All those mountains which look big to your naked eye will be quite small through an ultra-wide lens, and the wide angle means you get huge amounts of foreground in your shot, so you have to put something - anything - in the foreground to avoid it looking unbalanced. Hence the origins of the "put a rock in the foreground and it will be OK" school of landscape photography. It can look very good if done well, but it can also look very clichéd.