Maybe Battle.Net. I might request my account data just to see the creation date now.

Oldest accounts would be AIM and Hotmail had I held onto them. Some friends still use their old Hotmail email and take a lot of crap for it. I still remember my Hotmail password. It was four letters.

Then came FB, Gmail, and Steam all around the same time. I still use Gmail and Steam every day. FB not so much.