First, the TIM's primary function is to fill the micro-gap between the CPU and the heatsink. Witouth TIM, there will be air trapped between the surfaces, and air is one of the best heat-insulator you can get..
So, when you took of the heatsink, you were already fucked, because when you reseat the heatsink, air bubbles will form under the heatsink.
ZP is a cheap, generic paste around 2-3$, it will do a job, but a little more expensive paste (5-10$) will keep your CPU 5-6 C° cooler. As people said before me, a premium paste only give you 1-2C°, doesn't worth the money.
The best stuff, you could use as TIM is liquid metal, but that is expensive and dangerous, as it is electrically conductive, it could short your PC if it's accidentally spreads around.
For application, just put a small blob or line in the middle of the CPU, and put on the heatsink with one, quick move. If you put too much paste, it will actually insulate your CPU, most TIMs are not that good at transferring heat, just better than air.
Thermal conductivity(W/k*m): copper: 401 > aluminium: 205 > Generic TIM: 4-8 > air: 0,002
Some TIM will last forever, others will dry out over time and lose performance. But they should last at least for a year. Watch out for the temperatures, and re-apply the paste, if your CPU is getting hot.