For landscapes, you probably don't want a telephoto. The 18-55 isn't necessarily the best lens out there for what you want to do, but of the three you have listed, it is the only one that is not a lens going from telephoto all the way in to super telephoto territory and thus probably the only one of the 3 that would really be ideal for landscape. It is the APS-c equivalent of the 24-70 on a full frame camera, which makes it a fairly standard walk around focal length range (unless I'm shooting from across a room or doing head shots, my 24-70 rarely comes off my camera).
Relatively tight portraits could make use of the longer focal lengths, but it would be tricky to get far enough away to take head to toe portraits with either of the longer two lenses.
While you said you need a zoom, why do you need a zoom? You might be better served with something like a nifty 50 which is both cheap and outstanding quality (far better than what you would get with any of the 3) in addition to being workable for both portraits and landscapes.
I'd suggest you try re-evaluating your lens options based on what focal lengths you determine you need for the subjects and types of shots you want to be able to produce.