Code completion key bindings in Emacs

As Trey Jackson mentioned, hippie-expand is the way to go, but along with binding it to M-/, I also like having the TAB key do all my completion work for me. So I have this from the Emacs-Wiki in my .emacs file:

;;function to implement a smarter TAB (EmacsWiki)
(defun smart-tab ()
  "This smart tab is minibuffer compliant: it acts as usual in
    the minibuffer. Else, if mark is active, indents region. Else if
    point is at the end of a symbol, expands it. Else indents the
    current line."
  (interactive)
  (if (minibufferp)
      (unless (minibuffer-complete)
        (hippie-expand nil))
    (if mark-active
        (indent-region (region-beginning)
                       (region-end))
      (if (looking-at "\\_>")
         (hippie-expand nil)
        (indent-for-tab-command)))))
(global-set-key (kbd "TAB") 'smart-tab)

You could have hippie expand settings as follows:

;;settings for hippie-expand
(setq hippie-expand-try-functions-list
       '(try-complete-lisp-symbol
         try-complete-lisp-symbol-partially
         try-expand-dabbrev
         try-expand-dabbrev-from-kill
         try-expand-dabbrev-all-buffers
         try-expand-line
         try-complete-file-name-partially
         try-complete-file-name))

If you like completion of all kinds, I recommend M-/ and binding that to hippie-expand.

(global-set-key (kbd "M-/") 'hippie-expand)

It does a variety of completions, which are controlled by the variable hippie-expand-try-functions-list. In the .el files, you can set that to do the 'try-complete-lisp-symbol first to get the behavior you're asking for above, along with all the other expansions hippie-expand provides.

This would do that for you:

(add-hook 'emacs-lisp-mode-hook 'move-lisp-completion-to-front)
(defun move-lisp-completion-to-front ()
  "Adjust hippie-expand-try-functions-list to have lisp completion at the front."
  (make-local-variable 'hippie-expand-try-functions-list)
  (setq hippie-expand-try-functions-list 
        (cons 'try-complete-lisp-symbol
              (delq 'try-complete-lisp-symbol hippie-expand-try-functions-list)))
  (setq hippie-expand-try-functions-list 
        (cons 'try-complete-lisp-symbol-partially
              (delq 'try-complete-lisp-symbol-partially hippie-expand-try-functions-list))))