I have the Tamron 17-50 2.8 on my canon and its SUPER sharp. Pretty sure you can get it with a Nikon mount too.
I was going to say the Tokina but he didnt specify that he wanted a UWA lens. If that is the route the OP is trying to go though, I would say that Tokina 11-16 is the business.I'd look into some ultrawide angle lenses for your landscape stuff!
Some of the most popular lenses on the market right now are the following:
Nikkor 10-24mm
Tokina 11-16mm F2.8
Sigma 10-20mm
You'll be spending around $600-$800 but it will definitely give you a lot more than what you are currently using. Quality of glass in any of these lenses will also be a huge noticeable difference better than your current lens. If you don't want to spend that much, there are usually a couple floating around used!