Hello - you don't sp[ecify whether the lens is in addition to the 18-200 or will be used just by itself. This is important.
For a specific WA lens, something like the Nikon 12-24 or Sigma 10-20 would do (the Nikon is rather more expensive).
But think how much you are getting over the 18-200 at the wide end. If you will use the lens a lot, it is OK, but otherwise, Photoshop is pertty good at stitching these days (static subjects only, of course!).
For a single lens, I really like my 16-85. It is, in fact my "standard" zoom lens (I don't have an 18-200). The 16 is usually just enough for me and the image quality is excellent. But 16 is really not a lot wider than the 18-200, so there is a lot of overlap in capability at the wide end, and it depends on what your requirements are.
There are other brands (Tamron, Tokina) which could suit. I would recommend getting along to a store and trying them out - it's much the best way.
Nikon DSLRs, film cameras from Leica to Linhof