I haven't used Reaper.
I use mostly P3, despite my local stores not stocking it, so I have to order pots online. Partly I just find it to be really good paint, but it was also a process of elimination:
GW regularly discontinues colors. At one point they discontinued their entire range. If I want to be sure that I'll be able to keep painting models in the same color scheme a year from now, I can't risk buying GW paints. I don't care how good the paint is if I can't get more of it.
For the most part, Vallejo is fine. Overall I find their colors a bit on the translucent side compared to P3, and Vallejo pigment and emulsion seem prone to separating. Also, I hate the dropper bottles, because they waste paint (dispensing too much, especially when i just need a brush tip to mix into a recipe), prevent stirring (see the separation issue), clog, and generally get in my way. That said, Vallejo has a bunch of colors that nearly match discontinued GW colors, so I use a bunch of those. And while I love most of P3's colors, their metallics are terrible, so I also use Vallejo's metallics.
Dark Eldar plog: Drug-Crazed Space Elves
Stupid humans plog: Calyptra's Stupid Humans
Vampire Counts plog: Bat Country