In my opinion, from bad experiences, STAY AWAY from the wrap around insulators and the fin tube insulators. I have had to go back and replace countless numbers of these for myself and others on custom jobs. The best bet is a BULLNOSE type insulator as mentioned earlier. You will notice that many of the power companies now use this design to insulate guy wires on poles (a much larger version-but the same concept). On line posts, I like to use a pinlock insulator that is fairly robust. The ones that we have been using for the past several years were the Gallaghers. I have only had a problem one time with a few that were brittle after 6 years. They had a 10 year warranty and the farm store replaced them with no questions. They had the local rep take a look at them and he even gave me a few extras!
Another thing to keep in mind: It is HIGH-TENSILE NOT HIGH-TENSION. I say this so you will keep in mind not to overtighten the wire. There is no need to get this banjo string tight -- especially if you have power on it. In the old days when hightensile first became popular, the idea was to install 8-12 strands banjostring tight with springs and the animals would just bounce off of it. This did not work. We found that it needed to be electrified. If the animals are rubbing on it, there is a problem.
You might also consider making all of the wires hot. The neutral wires really dont serve a purpose unless they are hooked to a true hot/ground system where all of the nuetral/dead wires are tied back to the original ground system that the energizer is hooked to. Otherwise, make them ALL hot and if two wires get mashed together--no big deal.
A few years ago, we went to a university grazing school where the Gallgher rep was teaching and we thought he was crazy with some of these ideas. Have since learned that they know what they are talking about... and have the people in the field to back it up. I have always been willing to pay more for warranties and service. You can always find places to buy on the internet, but it is always better to buy from an actual store where the folks that work there can assist you with questions and demonstrate how some of these products work. Doing business with real live people, in person, makes sense.