It depends on your speakers and your amplifier. For that long a run you obviously want a cable that has low resistance, low inductance, and low capacitance per unit length. How low is low enough will depend on speaker impedance and amplifier output impedance.
I can't offer specific cable suggestions for that long a run, but the following technical considerations should allow you to rule out a lot of unacceptable choices.
At the end of this post are links to a wire gauge table and an inductance calculator. The inductance calculator applies to a straight piece of solid core wire, but I think it can be used as a reasonable approximation for most cables which have ordinary, non-exotic construction. Of course, if inductance per unit length is provided by the manufacturer of any particular cable you are considering, use that figure instead.
Keep in mind in using this table and calculator that the length of your run is 100 feet, the length of the round-trip the signal has to make.
The gauge should be large enough to keep total resistance to a very small fraction of the speaker's impedance at the lowest point of the speaker's impedance vs. frequency curve. Total resistance should also, preferably, be low enough to not significantly add to the amplifier's output impedance (which is the amplifier's specified damping factor divided into 8 ohms).
Once you have chosen a wire gauge, determine the inductance either from the manufacturer's specifications for any cables you are considering, or, approximately, by using the inductance calculator. To use the calculator, you will have to input the wire diameter indicated in the wire gauge table, for the gauge you have selected.
Then calculate the inductive impedance at 20kHz (the worst case frequency) using the formula 2 x pi (which is 3.14) x 20000 Hz x L, where L is inductance in Henries. The result of the formula is impedance in ohms. That result should be kept small in relation to the worst-case speaker impedance, and the amplifier output impedance, similarly to how total resistance was kept small.
Then give preference to cables which meet those criteria and also have the lowest capacitance.http://www.powerstream.com/Wire_Size.htmhttp://www.consultrsr.com/resources/eis/induct5.htm