As a general rule (there are exceptions!), prime lenses (like the 50mm) are sharper than zooms (like the 18-200) and zooms with a smaller range tend to be sharper than the so-called superzooms.
To see which focal lengths you use most, have a look at the EXIF in the photos you've taken with the 18-200 (right click on a thumbnail, choose Properties and look in the Details). Once you know that, you might be able to decide which range of zoom you want (first...) Personally, my tastes run to the wider angles so I use a Nikon 24-120 alongside a 12-24 Sigma in my main kit (with a 70-300 as long cover) but I know people who prefer the longer end so have 70mm as their starting point (weirdos!!!
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Having said the above, I'm currently trying to lighten my kit and have bought myself a Tamron superzoom for my old D70. This goes a bit against my grain since I prefer shorter range zooms but the lightness of the D70 + 18-270 compared to the D800 and short range zooms to cover the same range has persuaded me to give superzooms another try. So far, while ultimate image quality doesn't really compare, it's acceptable for holiday photos (to me). Especially given the reduction in cabin baggage weight allowances...
FWIW, the "Holy Trinity" of lenses for Nikons (as far as zooms go at least) are the 14-24, 24-70 and 70-200 (all f/2.8) but that kit needs fairly deep pockets! Cheaper alternatives exist and good copies of 3rd party lenses (always worth checking the actual copy you'll be buying to see if you're happy with it) from Sigma, Tamron and the like will probably be better than the 18-200.
