Jason-
If you have to have just one neck-shaping tool (other than usual planes, etc) I'd recommend the Nicholson rasp- pricey but good.
http://www.leevalley.com/wood/page.aspx?c=2&p=30288&cat=1,42524I use the #50 but it is very similar to the #49.
You could do a complete neck with the Nicholson and some sanding sticks.
I find that I also use a couple of smaller LV 'Hand cut' rasps (Ajax and Blundell) for some work around the heel and headstock.
http://www.leevalley.com/wood/page.aspx?c=2&p=20133&cat=1,42524A small round rasp can be handy around volutes, the other one I use is a half round-flat combo.
I also have a 'half-round' (it's really about 1/8 round) Microplane blade that fits in a hacksaw frame which works really well around the heel-
http://us.microplane.com/30010halfroundraspfor12hacksawframe.aspx.
Note- this is not the same as the blade in the 'handled' Microplanes, which I've found to be useless for wood - flimsy, slow cutting and they clog up very quickly. They work great for doing lemon peel zest in the kitchen, though...
I used to use sanding drums quite a lot for neck work , but not much nowadays, except for the curved transition from fingerboard to the headstock, when I use a drum in the drill press.
Cheers
John