C-a conflicts with bash's move to start of line. Not everyone would care, but that's one of the few bash shortcuts I personally consider important. I can't stand the default C-b either, it feels like an awkward reach to me and makes tmux seem to require way too much chording. I've recently switched to using C-j, which duplicates the enter key. I also changed next and prev page from n / p to C-n / C-p. That way I can…
There's always C-\ (I like it because it's opposite ends of the keyboard. Easy to find :) Might steal C-n and C-P, although C-[ C-] are tempting.
Interesting we both came up with C-\ for tmux, I like using C-a and C-b in emacs too much. Plus I forget whatever it does in emacs so it can't be important.