Some interesting points here, I enjoyed reading the discussion so far.
Personally, I tend to use many different types of paints (as you can see in a pic of my workspace below).
Using paints from different ranges has become a kind of hobby on its own for me I started a couple of years ago with Citadel paints and loved them. Before that I used only enamel paints that required a thinner to clean the brushes, making the whole process more difficult. So, understandably, using paints that can be mixed with water was a welcome change.
After a while I bought a couple of Vallejo paints. They have much better bottles and are mostly a good match for Citadel paints. I've had some problems with certain colors, where the pigment separated too easily and they required a lot of shaking before they could be actually used but overall, I like this range.
I also used Reaper paints and while these pots are a tad smaller, the quality of the paint, its consistency and coverage is very good too. Reaper has this nice idea of organizing its range of paints into triads, meaning you can buy three different shades of the same color that are designed to work together. And they actually do, you can get excellent results simply by using a three-step highlight system when you work with these. The other thing I like about the Reaper paints is that they actually have a small piece of metal inside them which helps a lot when you shake the paint before working with it. And once you've finished the paint, you can open it a get a nice looking metal skull
After that, I tried some P3 paints and while I love their consistency (they're a bit thinner than other paints I use) and excellent coverage, I don't care much for the pots they're packed in. They're hard to open and after a longer while the lids tend to get broken and fall apart. Still, I use some of these colors and I think they're probably the best for layering.
Another paint range I like a lot are Andrea paints. They come in boxes of 6 dropper bottles. They're a bit thicker than the other paints I use but they're unique in the design. They're sold in specific color sets. My favorites so far are black, gold, red and skin sets. When you buy a set you get six shades of a given color and a small leaflet with instructions on how to work with highlights and add shades. These paints are fantastic for working on individual models if you really want to get smooth highlights without needing to mix the paints. They work well with batch painting too. I love this range and would recommend it to everyone.
Still, if I was to start the hobby or if I had fewer paints and looked for substitutes, I'd definitely pick the Amry Painter set. So far I've bought only the metallics, washes and a few random colors. The metallics are probably the best I've worked with, thinner than Citadels with equally good coverage. The dropper bottles are a much better solution (especially when it comes to washes).