I have used just about everything for negatives. I list some examples here, with an example of at least one gum print made from each kind of negative:
I prefer to work with color film, since it offers the most versatility and creative flexibility, but when I switched to large format I also started working more with black and white film, and now use a mixture of 8x10 FP4 and 4x5 and 120 Provia.
For multiple printings I often use more than one negative per print; in the case of tricolors, of course, there are three, but I often use two or three negatives for monochromes as well, to cover the different parts of the tonal range, and often use the separation negatives for tricolor printings to print monochrome with.
A negative on the thinner side prints best, in my experience, but the great thing about gum is that it is such a flexible printing process that with judicious printing you can print just about any kind of negative. Very dense negatives are the most difficult to print well, but I've seen very nice gum prints made from absolutely bulletproof negatives.
For digital paper negatives, I prefer to use Epson Presentation Paper Matte (formerly called Epson PhotoQuality Inkjet Paper). I have a separate page comparing this type of paper negative to a digital film negative and to a different kind of paper negative here.
I have only recently started exploring more sophisticated methods for generating digital negatives; I recommend Michael Koch-Schulte's method for this kind of approach:
http://www.inkjetnegative.com/images/RNP/rnp.htm
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