In general I've liked thin (20 to 28 awg) solid core wire for interconnects, and thicker stranded wire (12 to 14 awg) for speaker wire... although I have heard some good sounding solid core speaker wire.
Silver wire sounds different from copper, and silver coated copper can sound like the best of both worlds, or the worst of both. Dielectric also makes a difference with interconnects in particular, with cotton sounding one way, teflon another, and PVC yet another.
The most recent ICs i've made used Jupiter 23 awg 5N copper with cotton insulation for the + lead, and Neotech 20 awg UP-OCC copper with teflon insulation for the - lead, and Eichman Bullet RCAs. They sound great to me... and outperform most commercial ICs I've tried. Got the wire from partsconnexion.
Neil