Depends on budget and what you want to setup you lenses for eventually.
I'd not recommend the super zooms. 18-55mm, they are a duplicate to what you already have (and maybe worse off in picture quality). At the longer 55-200mm portion of things they are not fast, and you might as well just get a dedicated 55-200mm telephoto which typically will give better quality w/o duplication of focal lengths to the kit lens.
A 55-200mm would also allow you to do landscape and portraits using longer focal lengths (more subject isolation, tighter FOV, further working distance). You'd not benefit from fast aperture or shallow DOF of course.
A Tamron 28-75/2.8 or Sigma 24-70/2.8 are viable choices as well for a low price. You gain the benefits of faster aperture and short telephoto FL. The bad is that it over laps your kit lens in some FL.
A 50/1.8 or 85/1.8 prime would do very well as portrait lenses. But these are not zooms, so you need to be comfortable (or be willing to adapt) with primes.
Another choice would be a Sigma/Tamron f2.8 90mm/105mm macro lens. Not too expensive, fast, works well for portraits, and gives added opportunity to do close ups and macros. It does not over lap your existing lens as well.