If only the original early-mid-80's formula Tamiya acrylic was still available...
This was a paint which had amazing coverage and self-levelling properties. You could one-coat, brush paint a model straight out of the bottle and an observer would be hard-pressed to tell it wasn't airbrushed without close examination. Those who can remember it will know that this is no exaggeration. Even that most difficult of colours, gloss white, could be slopped on (to an extent) and you could count on it levelling out to a glass-smooth finish.
The current formula can still be brushed, with care, but works better in an airbrush. Tamiya paint has a fair amount of latitude when it comes to thinning for airbrushing - I have gone up to 6 parts thinner to 1 part paint with no appreciable degradation in paint adhesion - it just works. Their flat paints dry absolutely dead flat.
As others have mentioned, it has an incredble shelf-life and I still have some bottles from the early-mid 80's.
On the downside, they have a limited colour range, most of which aren't matched to any particular paint colour standards, and though still listed as available, they seem to have done away with the larger 23ml bottles, which were great value for money, and replaced them with the "mini" 10ml bottles, while still asking the same price (thats what they did here in Aus anyway).
Gunze have a wider range of specific colour matches and airbrush very well. Brush painting is about on par with the current Tamiya formula, and the same rules apply. Like Tamiya, it tends to "skin" very quickly, so don't brush back over a previously painted area, or you risk "roll-up" When fresh, it's thinner than Tamiya, so brush painting usually needs more than one coat. It can be airbrushed straight from the bottle, but does work better with a little thinner. Shelf-life I would rate as good as Tamiya's, I have some unopened bottles which go back to the late 80's which appear to be as good as new.
Here in Aus, Tamiya and Gunze acrylics are about even on size, price and availability and you can often find them displayed side by side. It pretty much comes down to personal preference (I'm more familiar with Tamiya's properties) or which brand has the ideal colour match for whichever project I'm working on.