breaking strength or tensile strength is the the most important parameter. Polipropylen/polyamid ropes are generally better than natural ones (like manila or cotton). Knots reduce the tensile strength, but if you get a rope that can hold 800-1000 kg it hardly matters.
From what I've read a thinner rope is more effective (it will make you pass out faster) but causes more pain as it digs into the skin. A wider rope is more comfortable. 1 cm +- 3mm should be fine, I think. Also make sure it doesn't stretch.
As for the knot positioning, most guides tell you to put the knot at the back of your neck the way that when it tightens it slides to the left. It's supposed to be the most effective way. Geo Stone in Suicide and Attempted Suicide argues that you should put the knot at the front of your neck to avoid crushing the windpipe (which causes a lot of pain and suffocation).
Either way should work, after all you're putting your entire body weight on the rope.
Again I'm no expert, just repeating what I've read during my research. Too bad the people who actually died from this can't give us any tips.