Usually, my starter is my strongest.
And logically, that makes sense; your starter is your first Pokemon and has been with you the longest, assuming you don't box it. So it would of course have the most experience, or at least have been in a few more battles than the rest of your team.
Personally, I keep my starter a level or two ahead of the rest. Just a thing i like to do. I train everyone equally, and I try to get everyone to the same level give or take a level while training, but I tend to let my starter be just a liiiiiiittle higher. A small tip of the hat for being with me so long, you know?