I personally use a mix of artist acrylics paints, GW paints, cheap modelers paint, Tamiya and uh... basically whatever paints I have lying around, and my secret weapon: Johnson's Future Floor Polish (Or as we like to call it in England Klear), which is essentially transparant acrylic.
This basically opens up a wide range of options when painting, it allows you to extend the life of the paints you have, mix up superior washes, varnish what you've painted and well, pretty much everything you could ever want!
Heres an article: http://www.paintingclinic.com/clinic/guestarticles/magicwash.htm
Do a search for 'Magic wash' for more info though, it really is limitless.
The best paints are probably Privateer Press' P2 Paint, and Vallejo model and coat d' arms (Who used to make GW paint until they decided to make their own colour), but those are bout the same price as GW.
The main thing to consider is that if you treat your paints well, and use them with a little restraint they will last you a very long time (Appart from things like white and black, but you can afford to skimp on those ones
)