For passive monitors (without built-in amplifiers) use 12 gauge speaker wire. Your local music store probably has some in bulk: tell them how much you need (measure first) and you're on your way. Thicker gauge (lower #) wire is the equivalent of a bigger hose for water. It will sound better and increase the distance you can run it without losing high frequencies (but keep your runs short, of course).
For powered monitors, I use mic cables. Three connection ends (XLR, TRS) are better than 2 (TS, RCA).