I've found that if I select the last image in the sequence and shift-select the first, then the last image (most-selected, in LR terms) in the sequence is on top. The reverse is also true. And as mentioned, you can manually select any image to be on top of a stack, or click and drag it to the front if the stack is expanded. Don't forget you can also 'Auto Stack' images by capture time, but if you're trying to stack images after having made an HDR of the group that probably won't work for you.
Mike.