for nikno, theres a nikkor wide angle. then theres a tokina 12-24, a tamron and also the sigma 10-20. all have advantages and disadvantages. if u google, there was a shoot out between all 4 i think. optically wise, i think nikkor and sigma are top of the list.
but dont get it in your mind, that you need a super wide angle to shoot good landscape photography. some really good landscapes can also be shot with telephoto zooms... or 85 or 135mm primes... all down to how u like to shoot.