GW - most expensive in any way you look at it, since it comes with 50% less paint than everyone else (12ml vs 17-18ml). Also, that bottle... Paints are pretty good, tho
Vallejo - IMO, same quality as GW, easier to get (no restrictions on selling), comes with 50% more paint than GW, and in droppers. Washes and metalics, not that good tho. Outside the GW/Warhammer bubble, its by far the most popular paint out there.
Army painter - cheaper than above, somewhat inferior quality, separates like hell, but works like a charm on aribrush, so, you might consider that. Pretty damn good silver metalics and washes that ara as good as gws. (Better, if you dont like too much tint on the base color). Harder to get by than vallejo, but at least, no restrictions like gw.
REaper- I really cant get used to reaper paints overall, their finish bother me.
P3 - more expensive than vallejo, cheaper than gw, better than both. Too bad the bottle is not a dropper
Scale 75 - The real deal. Hands down the best paint brand i have on my bench.