I use Vallejo Game Color (GC) and Model Color (MC) paints mostly. The MC paints have been around the longest as the paint of choice among high quality painters. It's all about color choice, and high pigment strength. The GC are made to be more durable, and specifically match the Citadel paints color for color. So, I see it as a way to ween people away from the Games Workshop crowd.
I've got the new Reaper Master Series paints, and I like them. I bought the metallic golds and silvers, and the medium and light flesh color sets. The color system of sets of 3 is a wonderful time saver. I like the metallics--they have a nice sheen to them in the light, but can appear dark otherwise. I've been on a NMM kick lately, but will give a thorough testing of their new metallics on my next few minis.
The convenience of dropper bottles is a must for me. Messy paint lids are just a time waster. I've stayed away from GW paints ever since I had to open one of their jars with two large pliers--it is not an experience I would want to wish upon another painter. If you like the Citadel colors, go with Vallejo's GC instead, and you won't regret it.
I also use a few choice Liquitex artist colors and a mixing chart to help me get certain custom colors that I cannot find from other brands. It has helped me learn about color mixing and blending a fair amount. For those serious about getting exactly the color they want, I would highly recommend Liquitex medium viscocity artist acrylics. Most colors are $3-5 for 2 oz. of paint, while a few colors are up to twice as expensive due to the pigments used. If you are especially choosy, avoid any "hues," as they tend to imitate a more expensive paint color using cheaper pigments, but if you are price conscious, then that is exactly why they made hues.
Liquitex paints are thicker than other paints. You will get more out of their 2 oz. bottles than most 4 oz of the brands more common to figure painters. I thin down my Liquitex with mostly airbrush medium to a consistency I like, and then put it into a smaller dropper bottle. It has proven an excellent addition to my paint collection.
If I were to start anew, I still think I would go for Vallejo Model Colors. Reaper's new Master Series would be a close second choice. For newbies, I would probably recommend they start with the Reaper Master Series paints.