The best drinking Coconuts are when then they are young. A this stage there is very little Coconut "meat" on the inside (2 mm) but the juice has a much nicer taste. By the time the meat has built up to its full thickness (about 1cm) the milk (juice) has started to taste a bit bitter and is not so sweet. Unfortunatelly they do not normally sell young Coconuts in grocery stores.
One little known fact about the Coconut is that when it falls of the tree and lays on the ground for a while the inside fills up with a spung like substance that you can also eat. When I was a kid this was my favorite way to eat the Coconut even though I did not find them at this stage very often (normally they were harvested and did not get to this stage) At this stage there is normally as small shoot growing out of the Coconut. This is because the Coconut is actually the seed of the Coconut palm.