Switching between the two most recent buffers is something I do often enough to warrant its own keybinding:

;; Toggle two most recent buffers
(fset 'quick-switch-buffer [?\C-x ?b return])
(global-set-key (kbd "s-b") 'quick-switch-buffer)

On my Mac I have bound super (s) to the option key, which opens up a whole world of new possible keybindings.