I have a Nikon D60 with 18-55mm which is a terrible lens, it's not sharp at all. I mostly shoot landscape and I really need a sharp lens. I am considering to buy one of these lenses. Sigma 10-20, Sigma 17-70, Nikon 16-85, Nikon 18-105, I appreciate any suggestion or comments, thank you

In ultra-wide zooms, Tokina rules unless you want to spend way too much money on the Nikon beast. I have the older 12-24 f4 which is quite nice. I have used it with a D70s and now a D90. So for crop camera it is equal to a 16-35mm in 35mm terms. The newer Tokina is the 11-16 f2.8, it's a stop faster but smaller zoom range. I hear it's quite nice too. The 12-24 can be had in very good to mint condition for about half what I paid for mine new 4 years ago.

I shoot a lot of landscape now with both D90 and F100, I use a prime 20mm f2.8 and a 35mm f2 both are Nikon AF-D. They are both wonderful lenses, VERY sharp and small and lightweight for hiking. I am a bit fan of prime lenses!! And they hold their value better, imo than the cheaper zooms.

I also have a 24-85 AF-S that is VERY sharp and makes great colors. Sometimes when I really want a zoom I use that on the F100, but that makes a pretty heavy rig for hiking around.

Why do you say it's not sharp at all ? mmmh... since you shoot mostly landscapes I guess you do not need to be quicker than f8. And at f8 and higher I doubt there is significant sharpness difference with the models you mentioned. I can be wrong of course... Now if it was about distortion I would understand but I would say if you don't have any tripod it is better to put your money on one rather than buying a lens that won't give you very extra sharpness you are after.

I have the same camera and the same lens, and I have to agree with Nazir. Actually, I was beginning to think it was me, so I'm sort of glad to hear he has the same issue! However, I'm also starting to believe that part of the issue relates to the D60 autofocus itself, which isn't very fast or accurate in certain conditions. My other two lenses, though, a 55-200VR and and old 28-80 with manual focus on my D60, do not have the same sharpness issue. I can use those two lenses under the same lighting and get sharper images than my 18-55VR. I've also experimented with turning off the VR, but it makes no difference. Essentially I find that the 18-55 is acceptable under ideal lighting conditions where I can go F10 and, say, 1/125 or quicker, and the zoom in the middle range. But, even then, I've never been able to get the tack sharp images I want. Basically, its a crappy kit lens.

I'm surprised that the Nikon stock lens is terrible. I had the same lens with my first DSLR D40 but never had any issues with it. I also agree with what has been said before about a good sturdy tripod for your landscape photos so take a look at Manfrotto tripods. Ther are quite expensive but believe me they are really good and durable so your investment will pay off in the long run. As for the lens your selection basically revolves around Sigma and Nikon. First of all, if you want to shoot landscapes listen to what Clyde said. Prime lenses are the best and they are also great with portraits. If you really want to have a versatile lens with the zoom for your landscapes I would rather go for Nikon lenses and not Sigma but that will cost you more. Anyway, good luck with your bargain hunting and landscape photography.

Jacques, I've used the 18-55 before and I think sharpness is fine. Actually I don't think there's any noticeable difference on any zoom lenses in terms of sharpness, but I can understand why he thinks so, the lens just feels plastic and tacky.
Nazir could you show us an example of the kind of sharpness you want to achieve?

I am not saying the 18-55 is the sharpest lens out there. Of course it is not. But I doubt that the models Nazir proposed will give so much extra sharpness for the kind of shot he does - i.e. landscape, static, for which the use of a tripod is the way to go to gain sharpness in the first place, whatever the lens. It is not as if you need to shoot fast - say at f2.8 or f4. Then of course considering another lens would be a must.

Now if you feel that sharpness HAS to come from the lens and want the best I second Clyde about the primes. Getting some second hand AI(s) Nikon prime (either 20/24/28 or 35) is the best pure-lens choice and not the most expensive one (you don't need AF for landscape don't you ?). Or you can go for a much more expensive Nikon pro lens zoom if you have the money. But I feel this is not what Nazir really need right now. Just my $0.02