Use them ALL wisely, and notice the modify>convert menu.(master the converting options and it WILL yield rewards)

Nurbs: armors cars, metallic hard surfaces... (try to avoid patch modeling, SubDs can make your life easyer )

Polys for all things as you can make a general shape then proxysmooth it, or even better subD it refine and then if you want back (also see CPS plugin) (also the proxysmooth is grat for rigging too)

SubD is the best thing to work with, if your graphic card is up the task... It has the best of both worlds but is the slowest to visualize.... and is the best thing to use when modeling realistic faces/bodys

The gratest problem with polys subds is UV editing, nasty stufff, personally i just auto project it to 4 planes then relax them all and then forget abut them as i project all textures and bake them to files, then project more, then bake...