Obviously the normal ones are: Ctrl-x, Ctrl-u
Alt can be a good leader prefix. Alt-x is emacs-extended-command. Alt-m is used by spacemacs.
Visual Studio has Ctrl-k, Ctrl-e, but IMO, they are not good. Those are very "hot" keys that are used all the time
Tmux has Ctrl-b, which is almost always rebound to Ctrl-a for memory compatibility with Screen's Ctrl-a. It's also not good IMO, but Tmux is used very often that it is reasonable to include Ctrl-a as a leader key.
One I've started playing with is Ctrl-Insert - it has almost no conflicts. There are also Ctrl-Numlock and Ctrl-Scrolllock