Lux. If you've tried their coffee, pastries, or other assorted goodies then you are likely a believer. Yes they are are always busy. You will likely have to wait in some sort of line, depending on when you go. Your patience will be rewarded by your taste buds singing praise for a job well done. Most of their drinks tend to run less sweet (which isn't a bad thing) but they provide a variety of sweetener options to enhance the flavor to your preference.
I recommend picking up coffee beans if you have a grinder, or alternatively, I believe they can grind it for you. This is truly a gem in Phoenix and without a doubt, my favorite coffee joint.