Vallejo:
In my opinion Vallejo are simply the best acrylics out there with a truly huge range.
They make the model colour range ( my personal favorite) that have over 200 different paints all in 17ml drop bottles. These paints are better for a more realistic pallet as they are not as bright as many of the game colour range.
The game colour range, which are essentially game workshop paints but in dropper bottles and the better Vallejo paint consisency.
The Model Air range, which is designed specifically for airbrush use, though they can be used with a brush too.
All three ranges have hundreds of great colours and at a very good price and you get 17ml of paint compared to the citadel 12ml!
They also have a rather cool range of special effects paints too, from the fluorescent range and the smoke, woodgrain and more special paints for loads more effects.