I agree with @Tyler
In my case I use dwm as window manager, so meta (ALT) key is used to control almost all its features. When using emacs, meta key conflicts with some key bindings, eg: when I'm moving around words M-f or Mb (that's why I came to this thread ;)), so I found three choices to get rid with this issue:
1) change emacs key binding and use super key as meta key
(setq x-meta-keysym 'super
x-super-keysym 'meta)
Both symbols (variables) are required, because if you just use only x-super-keysym 'meta, it enables both keys as meta key. If you just use only x-meta-keysym 'super, it disables both and de command buffer will return s-x is undefined.
2) change dwm key binding for meta to super key in config.h file. take a look in http://dwm.suckless.org/customisation/windows_key
/* key definitions */
-#define MODKEY Mod1Mask /* meta (ALT) key */
+#define MODKEY Mod4Mask /* super (windows or cmd) key */
3) override xmodmap keymap bindings swapping mod1 to super and mod4 to meta (be sure to check what xmodmap returns at first glance, and not affect keybindings of other programs)
Finally, I recommend not to change default emacs meta key because is less comfortable to your fingers and may cause you to respond slowly to keystrokes. Try options 2 or 3 depending your environment.