People should re-read the OP and understand what's going on here.
The OP owns every official recording except these three early albums. Therefore, it won't be long before he/she owns those three also. What's at issue is not which album is "the best so that I only buy that one", but which album would be "the best to buy next".
It seems pretty clear that they are on a bit of a journey here, to discover the origins of U2 and their path to greatness. Therefore "the best" album to acquire next is absolutely BOY.