[DaleHCook]

I always keep Artic Silver 5 handy for things like this and computer projects.

That's what I would use for a CPU or GPU heatsink, but it is pretty expensive for a power semi such as your TIP50. My standard high-performance heat-sink compound is what is used by Nautel, the premiere manufacturer of AM and FM broadcast transmitters, for their power MOSFETs. It is AAVID Thermalcote 249G, which has much higher thermal conductivity than standard silicone compounds such as GC Type Z-9.