Of the companies I've built kits from I would rank them in this order.
Academy: some of the highest quality I've seen. lots of detail I couldn't tell you how acurate they are but at least it looks good.
Hasegawa: a little cheaper, but still amazingly detailed.
Tamiya: love their car models and their jets are very nice.
Revell: so so. For as cheap as they are their good kits, but they aren't as detailed as the kits above. Fit and finish can also be iffy.
Italeri: if you want something small and cheap that you can slap together in a couple days, this is for you. They have minimal detail but they do have some cool subject matter.
Testors: I love their paints but avoid their kits like the plague. Basically for the same price you can get any of their kits form Revell with about twice the detail and gimmicks and the parts will fit better.
Airfix: HAHAHAHAHAHA!! I can't believe it took them this long to go bankrupt.
edit: let it be known I don't know how acurate any of these things are. personaly I don't care as long as it looks good.