C-a a. Using emacs with screen for many years, this is almost automatic for me.
Moreover, I forgot if it's the default or if I set it somehow, but in Emacs a single C-a goes back to the beginning of a line, but C-a C-a goes back to the beginning of text on the line, which is almost always what I want anyway. So I press C-a twice anyway, everywhere, and I don't have the problem you described at all :)
defbce on
defutf8 on
escape ^]^]
markkeys "h=^B:l=^F:$=^E"
setenv LANG en_US.UTF-8
startup_message off
term $TERM
Copy pasted from somewhere, i do not recall from where.
ctrl-A is now ctrl-]
This does not clashes with Emacs, ctrl-A
As for editors, I use vim. (With almost no customization, as others have advocated.)
These days I use C-z for tmux. It works, and C-z z is easy enough to type when I want to put something in the background.