Pure ethanol (or everclear from states with higher % limits than california) and/or propylene glycol. Ethanol dissolves things the fastest, but they both do the trick, and PG is less gross to some people since its kind of sweet. I found there to be a big difference when making diclazepam tincture; It almost didn't seem to want to dissolve in PG without dissolving in ethanol first.
With Ethanol, you have to be mindful of evaporation if the container isn't super airtight, because that can lead to a more potent tincture than you intended. PG can leak pretty easily too, but it doesn't evaporate; it just slowly seeps out and gets on everything. Sometimes I mix the two together.