Out of curiosity, when you start a new solution and you add your first project to it, if it's a window's form application, you get a nice and neat form called "Form1" (how creative!). Do you rename that file? If so, what do you name it? If not, why?
I always name the main form some variant of 'Main', because that's what it is. All other forms are named by function. Exactly what the name is depends on the naming conventions set forth in the local coding standard. If I use mine, then it would be 'frmMain'. In the company for which I work, it would be 'MainForm' in the old naming convention I used several years ago, it would be 'o_Main'. I don't refer to it simply as Main() because that is the name of the entry point in Program.cs, and would thus introduce ambiguity, but it is actually a form and should thus be named according to the naming convention for forms as per your particular environment. If you don't have a coding standard, you should really set one.