I've used/use Kimera, Scale 73, Citadel, Army Painter, Vallejo, Green Stuff World, P3, War Colours, Createx (airbrush) and a couple of local brands.

Createx is great, but aside from that I think Vallejo is going to be the only brand I buy going forward except for the citadel contrasts that I really like.

Not because Vallejo is better than say Scale75, but because my 10 year old Vallejo is as good as it was when I bought it and all the other brands tend to break down to the point where you can't mix the medium and the pigments on all colours after a few years. Citadel doesn't do this, but like half their colour range turns into cement instead. :p