If you want to photograph really good landscapes, get a wide or ultra wide angle lens. In the past, I've used non-OEM lenses such as Sigma but due to inferior quality (and thus much lower cost), I replaced all of them with OEM lenses (meaning lenses with Canon or Nikon brands, same as your camera). A wide to ultra wide angle lens that covers 16-24mm zoom range mounted on a full frame camera is the best choice for night or aurora photography.
In addition to zoom range, also consider lens speed. Ideally, buy a fast lens (especially if you decide not to get a full frame camera). A fast lens typically has a 2.8 or smaller f number - when you shoot in dim light situation, the fast lens allows you to get more light into your camera and therefore get better images. The best wide angle zoom lenses are Canon 16-35mm F2.8II and Nikon 14-24mm F2.8G.
I'd suggest getting a zoom lens instead of prime lens (with fixed focal length) for your first landscape lens, so you can play with composition. However, prime lenses typically are €faster€, smaller and lighter (because of simpler build), and have better performance in low light condition. They are typically very good for portrait as well.
The following recommendations are mainly used for landscape photography.

CANON LENS RECOMMENDATION
If you already own or decide to buy a Canon camera, this is my lens recommendation (in this order).
€ Canon 16-35mm F2.8 II USM (Canon's best wide angle zoom lens)
€ Canon EF 14mm F2.8L II USM
€ Canon 17-40mm F4L
€ Canon 10-22mm F3.5-4.5

NIKON LENS RECOMMENDATION
As mentioned above, Canon and Nikon typically have comparable products, no matter for cameras or lenses. Following are a few wide angle lenses for your consideration if you own or decide to purchase a Nikon camera.
€ Nikon 14-24mm F2.8G (Nikon's best wide angle zoom lens)
€ Nikon AF-S Nikkor 16-35mm F4G ED VR
€ Nikon AF-S Zoom-Nikkor 17-35mm F2.8D IF-ED

SONY LENS RECOMMENDATION
Sony's mirror less cameras A7 and A7R brought the entire industry to a new hype in 2013.
However they have limited lens choices compared to Canon and Nikon.
€ Sony SAL1635Z 16-35mm f/2.8 ZA Lens
€ Sony 16-50mm f/2.8 Standard Zoom Lens

NON-OEM LENS RECOMMENDATION
Note: Each non-OEM lens will specify whether it's made for Canon or Nikon. Please read carefully before purchase.
Non-OEM lenses are a lot cheaper. Major brands include Tokina, Sigma, Tamron etc. Those manufactured by Samyang under the brand names of Samyang, Bower, Rokinon offer a great value for about 1/3 the price of the comparable Canon and Nikon lenses. Test these lenses thoroughly after purchase, as quality control is notoriously inconsistent. The Zeiss 21mm F2.8 is a great option, albeit a very expensive one. As I'm not using any of them now, I can't give you a specific recommendation.

Below is my research for your reference only. They are not listed in particular order.
€ Tokina AT-X 16-28mm F2.8
€ Tokina's 11-16mm F2.8
€ Rokinon / Samyang 14mm F2.8