Long-life unsweetened soya milk. Current source is only 50p per litre. It's got a few added vitamins and minerals to give it the same kind of profile as cow's milk, which is useful for vegan-leaning people such as myself. The best soya milk I can get in the USA is nothing but soya and water, but it tastes much the same.
I've not tried the other types much. I don't like sweetened or flavoured soya milk, and the kinds they keep refrigerated even before opening are too expensive for me to take seriously.
I used to have trouble with soya milk curdling in coffee, but I stopped drinking coffee anyway to stop my teeth staining so readily, so that's no longer a problem.