For me, the D60, hands down. The main issue I had with the D70 family was that the histogram and highlight indication only works on the green channel, which has to be the daftest choice as it tend to be either the red channel that blows out first (i.e. skin) or blue (i.e. sky). This, given the D70's very protective attitude in metering to protect highlights, often meant that the shots that I took were underexposed - you can recover this to some degree after the event in post processing but it does mean that you are not using the full dynamic range of the camera.
Sure, the D60 doesn't autofocus with AF-D lenses but it does have a manual focus indicator so all those lovely old Nikkors are available for use and it is so small that you can carry it around with you more than one of the bigger cameras.
If you do want autofocus with AF-D lenses, access to the creative flash system etc I would recommend the D80 if you can spring the extra cash. The metering is criticized for being a bit "hot" (i.e. overexposes a bit in certain circumstances) but at least you can see that the exposure is wrong after you take the shot. The rear LCD is also far more usable that that of the D70.
I can't comment on the D100 but bearing in mind that the D70 effectively made it obsolete, personally, I wouldn't bother.
