Electrically conductive thermal paste
I hardly can see why you need a conductive paste? The aluminum is covered with thin insulation layer of Al-oxide and will break the electrical contact anyway with the time, regardless of the paste properties.


If you need the heat sink to be electrically connected to the element - use a wire instead and screw it on the heat sink.
Use copper heat sink and solder the element on it - this way you will get the best electrical and thermal contact possible and very effective heat sink at the same time. (Use very thin solder layer).