In the US most box wines are truly dreadful. In other countries there are some excellent box wines. The closest is in Canada, for example Ontario has some great red box wines. Try Peller Estates French Cross (it would go for 5x the price in the US and is a great example of the combination of vinification technique and new hybrid grape varieties).

For "regular" wine, my current favourite for price/performance is Ch. Potensac (owned by Leoville-las Cases). Drinkable young (ie. at 5 years) but great at 20 (if you can cellar it that long).