For most amateurs the 18-200 lenses by Nikon, Sigma and Tamron are
reasonable performers.
Any of these lenses outperform the Nikon 55-200 in that focal range. However
Nikon did recently add IS to their otherwise execrable 55-200.
The Nikon 18-200 is the most desirable because of image stabilization but it
costs twice as much as the others, is difficult to find, and its optical
performance otherwise is not much different regardless of what Nikon
partisans say. These lenses all distort at the wide end, which may not be
noticeable to some users, but the distortion is correctable in Photoshop. I
love Nikon but you pay a premium for the name tag that does not consistently
translate into performance that justifies the price for the purposes for
which the lens will actually be used.
Sigma announced a new IS 18-200 last September but never actually produced
the lens. The last time I checked they had not given retailers a release
date and they had not said they were not going to make the lens. Vaporware
marketing as in the computer industry. Check out the Adorama web site on
this lens.
A wider angle lens is desirable. Again the Nikon 12-24 is more costly than
the Tokina but the Tokina may be better optically. In fact the Nikon and the
Tokina may share more than an identical focal length and fixed f-stop.
Lenses longer than 200mm (equal to 350mm in 35mm film SLRs) for dSLRs are
infrequently used by amateurs in the real world. If you don't know why you
need one you do not need one.