Google. I use Chrome, so I assume it’s the default. I don’t think I’ve ever bothered changing the default on any of my browsers. Let me check. Firefox goes to the Mozilla Firefox start page, which is a Google search bar. Safari goes to nothing. Just a generic page with a bookmarks bar on the left and some top site and their icons on the right.
My browser is set to NOT OPEN a page when it is started; if I do ⌘-N to make a new page, it is blank until I type something in the address bar or select something from the Bookmarks menu.