Personally I use ctrl-^ as my screen escape character. This seems to be the one character that doesn't get in the way of anything for me.
You definitely want to change it from ctrl-a because this is beginning of line in emacs and many shells such as tcsh and bash. Hard for me to imagine why this is the default in screen. In fact avoid ctrl-any letter.
You don't want to use ctrl-space as this sets the mark in emacs.
You don't want to use the esc key as this is a lead in character for many emacs commands
You don't want to use ctrl-_, this is bound to undo in emacs (or at least it is in my emacs)
You may not want to use ctrl-] if you use telnet (few do these days) as this is telnet's escape character. This is my second choice of key.
Maybe you don't want to use ctrl-\, in my emacs it's bound to switching between buffers and as you said it might cause an abort (a core dump) in the right context. However, this would probably be my third choice of keys.
I don't know if it would work but maybe you could bind screen to use an obscure unicode character and then bind a key on your keyboard to transmit a utf-8 value when something odd was pressed like AltGr-Esc. You can use a tool like http://www.kbdedit.com/
to create a new keyboard mapping (on windows--similar things exist on linux/mac) and add a key comination to emit the unicode character. If someone tries this, I'd be interested to know if that works!