Use source control history to find out which parts of your system change most often. Compare with your regression test coverage.
I already know the answer to this since it's been a large process. Our most changed code is literally our front end. The reason is straightforward. We are redesigning it so that it will work as a Responsive UI. that means everything is getting tweaked around the front end. Our regression testing, therefore, is in the spin cycle. It's getting majorly overhauled. Our legacy interface, on the other hand, is doing well and will still be there for those who choose to use it, so that is adding an exciting challenge as well.
the biggest challenge I am personally facing is that the tests we have for our legacy interface are solid and they work well, but they are almost totally irrelevant when it comes to our Responsive interface. the IDs are different, the rendering code is different, the libraries that are used are different. the workflows are similar and in many ways close to the same but don't quite lend themselves to a simple port with new IDs. Thus I'm looking at the changes we are making and figuring out how we can best automate where it makes sense to. Needless to say, it's never dull.