Definitely the weaklies. But in all honesty, almost half my coins seem to come from either the Cryptarch, or postmaster, for when I do my first public event each day.

I always have a checkpoint at that exact area, as I use it to get that annoying 'kill 2 enemies with one fusion rifle shot 10 times' bounty. The sheer amount of thrall in that area means I usually only have to respawn once.

Always do your weeklies. 3-9 guaranteed coins a week.

my favourite spot to farm glimmer is at the checkpoint on the moon where you find the dead guardian, all you have to do is use ear wax that hive give you when they die and make sure not to kill the knight.