Ahhhhh, well that's a different matter entirely!
While I like GW paints, I hate the pot design and the price. My paints dried out and I had to replace them, which got expensive. I do have their foundation line and washes which I transfered to reaper/vallejo style dropper bottles.
I switched my main colors to Vallejo Game Color because the colors were closely matched to the GW range (in color and name), they came in dropper bottles, have not dried out in the 2 years I have had them, and were less expensive for more paint per bottle.
I have added some Reaper Master Series triads to my collection and like them as much as the Vallejo.
I use some Liquitex soft body artists acryilics and recently picked up their glaze and matte medium.
Every once in a while Testor's model master enamel and acrylics come in handy (more so when I was doing scale models, but still has some uses) as well as Tamiya paints (especially their clear red, and people swear by their spray primer which I have not used). I also have some Floquil and Polly S.
For Primer I use Krylon Fusion white and Rustoleum Auto Primer.
Sealers I love Testor's. Both Dullcote and Glosscote depending on the finish. I have used Krylon flat and gloss to good effect when I couldn't get Testor's.
If I need a dark basecoat, I use a flat black Walmart Generic Spray paint ($0.99 a can) sprayed over one of the aforementioned primers.
I have not tried P3 or artists oils. But may pick up some P3 in the future.