For orange zest, bitter, aka sour orange is usually recommended. I've put the zest of two bitter oranges in a 5 gallon batch of a strong beer with some residual sweetness and it was orangey enough that I don't think I'd ever add more than that. On a couple of occasions I've added the zest of two (Persian) limes. Those came out well balanced, with a hint of lime flavor/aroma.
When I add zest I put the zest in a hop bag, put that bag in a small jar and just cover it with vodka. Leave that to steep for a few days and add the zest and the vodka to my beer a week before bottling.