I want to build a 1:1 unun for a 6M dipole. Power level 100W. What core material should I use?
As Mike Waters points out, your application requires a balun since a 1/2 wave dipole is balanced and the coax is unbalanced. The purpose of the balun is to enforce equal current in both dipole legs. It performs this function by preventing current from flowing on the outside of the coax shield by presenting a high impedance path to exterior shield currents.
You can use a toroid form comprised of mix 31 or mix 43 material to construct a 6 meter balun. A 240 size toroid with three or four turns of coax wound uniformly through the toroid will prove effective. Wide, evenly spaced turns are important to minimize the interwinding capacitance.