The standard AR-15 trigger was designed to be used for military applications and I knew from experience I would soon tire of the poor trigger pull.
For my build of an AR-15 lower, I selected a Timney AR-15 Skeletonized drop-in trigger, partly because it looks so cool.
At $259.95 retail (you can find it for much less on line and save even more money by forgoing the skeletonized option), this is an expensive upgrade, but well worth it.