Like akerue said, googling will get you lots of info on FX and DX. Haha if there's a perfect lens, the manufacturers need not produce so many lenses, only make the perfect one will do cos nobody will buy the rest. What lens to use is all about the situation, no lens is perfect for all occasions.
There's no perfect lens, each lens has their own strengths.
In summary though, FX bodies utilize the entire lens whereas DX bodies utilize just the center 'sweet spot'. As of such, DX cameras have a crop factor, for Nikons its 1.5x(for Canon, its 1.6x. Canon 1D is 1.3x though)
In short... Fx = big sensor (equivalent to 35mm film), Dx = smaller sensor (for Nikon, 1.5 times smaller). As for the perfect lens - depends on what you are trying to shoot or achieve in the photograph, which, in almost everyone's opinion here, you still have a long way to go in getting to that point. Best to start researching on the basics first - like Dx versus Fx sensors or crop versus full frame, etc, etc.
maybe 12-300? and F1.4, built-in filters of all sorts, size as small as a 50mm 1.8D, internal focusing, VR mode, PC while allowing the user to choose whether s/he wants the PC to affect the lens composition, able to switch to macro mode, hm... maybe built-in speedlight?