So I've now used something like a dozen brands and lines of miniature and related paints, and I have opinions.
First off, a couple terms. Saturation refers to the "strength" of a color. Low saturation means closer to grey, while high saturation is very colorful. Coverage refers to how dense the pigmentation within the paint is. Paints with strong coverage are generally easier to work with for most purposes.
I've found a few paints that are all equally solid in my opinion: Vallejo's various model lines, Privateer Press's P3 (P3), and Reaper Master Series (RMS). Even among these, however, there are better and worse colors, and I've found experimentation is key to determining which is best for a given color.
Of Vallejo's lines, I've primarily used the Game Color (VGC) line, though I have some of the Model Color (VMC) and a few Model Air (VMA), specifically metallics. The VGC line is solid, but there's some weakness in the yellow/bone range, and there seems to have been a malfunction in their dark green, in that every one I've tried has been somewhere between a wash and an ink, good pigmentation but really, really thin. VMC is generally okay, though I've found the metallics from this line to pop nicely. The VMA metallics, however, are really impressive... except when they're not. The Chrome and Rust colors look incredible, but the Copper and Brass are painfully bad for their names, looking more like a tinted silver than either copper or brass. They might have a use, but it's something other than what they call themselves.
Privateer's lines seem to specialize in very vibrant, saturated colors. They have excellent colors across the gamut, though there's a little too much specialization in browns and olives, in my opinion--a few of the browns I have difficulty telling apart, and probably ten of their colors are olive or olive-tinted. I've yet to find one of their paints I was dissatisfied with, coverage-wise. None of this applies to their metallics, however. I've heard that the paint manufacturer managed to screw up all of P3's metallics in their first batch. I've further heard that the issues have been corrected, but that there's so much of the first batch floating around that all new orders for the metallics still get that bad batch. They are universally awful in my experience, which is really too bad in that they have some distinctly unique colors, such as Blighted Gold.
Reaper's Master Series line seems solid. It's not quite as consistently vibrant as P3, but it has a broader range, and the "Violet Red" is the best looking red I've seen in miniature painting lines. The metallics are good, but I have a hard time saying they're better than Vallejo's. They have a few different colors, however; the Old Bronze is a particularly nice somewhat green gold.
To be honest, I haven't used too much of Citadel's modern lines. I generally preferred the VGC line years ago, which has a color-for-color match to the Citadel paints, and even the problem dark green I thought better substituted by P3 Gnarls Green. I also have yet to try Reaper's Pro Series line. Per their marketing, they suggest using the Pro Series for base coating and the Master Series for further layers, much like the Citadel Foundation and VGC Extra Opaque lines in comparison to their respective standard lines.
Containers strongly affect the usable life of a paint. The worst offender is the modern Citadel line; its hard plastic containers with their hard plastic tops appear to allow more air in and let more moisture out than any other line I have. After two years in my kit, they're dried to solid lumps in their pots. By way of comparison, I have older Citadel paints (made by Coat d'Arms, now an independent) that are over a decade old that are still usable. Silicone tops appear to be the best at keeping in moisture; nearly all of my P3 paints are good, even after around five years. Dropper bottles appear to be very good as well, though I probably lost 1 in 5, and many 1 in 10 remaining are a little thick.
One trick I've found is moving paints from pots to dropper bottles, which can be purchased relatively cheaply. The transition is slightly tricky, but in moving 25 P3 paints to droppers I had no significant mishaps. You'll be left with a small amount in the pot, but it's not too hard to use that up. Bonus for that is you now have empty pots, which do have a use in my experience: wash containers. I have about a dozen washes of various colors in my empties, and the generally leak-proof nature of the P3 caps is hugely useful.
