The default tmux binding is ctrl+b, which is pretty awkward to hit on a Mac keyboard, because there's no right control. C-a was the default in screen, but that's the default in OS X for going to the beginning of a line (my shell uses vim mode, but it's inconvenient when ssh'd to other machines). You can't use alt as a meta key, and cmd+a is already used to select all text.
So - what are others using that doesn't override default OS X behavior that works well?
CTRL+b
awkward to hit for example and other people might have different ideas than you about convenient keybindings.