Of the 4 brahmaviharas, which have you found easiest to access in daily life and formal practice? Which has been the most difficult? 

For me, mudita (and joy in general) has always been by far the easiest and most natural, while equanimity is probably what I've practiced and studied the most and this has been extremely beneficial. Compassion has been the most challenging and what continues to be an area of relative weakness that requires considerable attention. I'm about a third of the way through Bikkhu Analayo's Compassion and Emptiness in Early Buddhist Meditation and it is good medicine for this.