Teflon is great for dielectric absorption and insulation, but its temperature coefficient is ghastly. It goes through a phase change right around room temp.
It's hard to find teflon caps above about 50 pF.
The second ADC that I designed was a dual-slope, and in ignorance I used a mylar cap as the integrator. It was terrible, several per cent error. A tech fixed it for me.
C0Gs are usually good. SiO2 (MIS) caps are superb, but are usually found inside chips. There are some for sale as discretes.