^Z because it's otherwise ‘suspend’, so it doesn't conflict with active use of anything else. (Plus ^Z^Z to send ^Z.)
I haven't checked, but if multiplexers don't already use XTMODKEYS to allow more complicated prefixes, they should.
Space for vim.
C-Space for tmux.
Especially with Ctrl bound to Caps-lock.
The tmux default is C-b which hasn't yet conflicted with anything for me (and I am very keyboard oriented). But the other suggestions might be just as good or better.
`C-a a` is so ingrained after years of first screen and then tmux use that I have to consciously not type it when using Emacs outside a multiplexer.
My leader key is the grave key. I’m not a big fan of using a modifier for something that can be done with one key press, so it’s perfect for me.
C-;
It's really C-o but I have that key combo mapped to C-; and capslock -> C when pressed with another key (otherwise defaults to esc)
Pinkies all the way
I use C-^ because yeah, C-a is emacs/readline 101 so I hit that 10s of times a minute.
C-a has always driven me nuts because it’s emacs/readline for “beginning of line”.
I’ve settled on C-o for a while but sometimes that gets in the way too.