1. If thermal conductivity is of primary concern, then Arctic Silver compound is the way to go (short of reflow soldering, which has far higher thermal conductivity than any of these Arctic materials), especially if the heat sink is anodized so that shorting the P7's slug is not a issue. Then you can use regular epoxy to fix the P7 in place.
2. Next best in terms of thermal conductivity is to use Arctic Silver thermal adhesive (epoxy).
3. Third is Arctic Alumina compound, and then fix the P7 in place with regular epoxy as in #1 above.
4. Last is Arctic Alumina thermal adhesive (epoxy). Many have had success using AA adhesive, so as long as you use a thin layer, even AA adhesive should be fine. AA compound and AA adhesive are also a lot less expensive than the AS counterparts. The AS stuff gives you a little more margin for error in terms of layer thickness.