I haven’t tried pistachio extract or essence, but Hector has recommended the La Cuisine pistachio essence.
I have one pistachio disappointment to report: Dumante verde noche (pistachio) liqueur. I had been eyeing this for a long time and when a local seller got it in, I took the plunge and bought a bottle, thinking it would be like a pistachio version of Amaretto. It does have a very nice toasted nut component to the flavor, but there is also a component that I just don’t care for- to me it tastes a bit like those chemical-y dyed red maraschino cherries. I keep trying to pair it with something that will make it taste good, but so far I haven’t found anything to get me past that maraschino taste (shudder).
If I were looking for a good quality nut flavor, I’d be tempted to use a nut paste (especially for buttercream, ganache or whipped cream). L’epicerie sells good hazelnut paste- intensely flavored and perfectly smooth. I haven’t tried the pistachio version.