none of them can play as a regista, but im putting the most solid player we have in front of the defence, with this formation we dont need a regista as much, tiago can pass and marchisio can surge forward where melo should do the simple stuff.
for me its either the
--------melo--------
-marchi----tiago---
--------diego-------
----trez------delpie
or prefferably
----melo-marchisio-tiago-------
----diego----------------------
--------------------giovinco---
-------------trez---------------