Least favorite is definitely invoicing clients. I have to have them done by the 10th, it's the 13th and haven't started for the 15th straight month. So I'm going to have to do them this weekend. I have about 15 that I need to do each month and 10 are awesome clients, but it's the 5 that suck that I have to fight with for 3 months to get paid that are the absolute bane of my existence.
One is trying to screw me on an 18k COs and another is trying to screw us on about 20k in COs. They are absolute trash clients and I want them out of my life.