Spike is also Twilight's much-needed reality-check from time to time. For example, Twilight needed Spike there to realize that she was being overly neurotic in Lesson Zero. Also, in Equestria Girls, Twilight was worried that the human counterparts of her friends would not accept her once they learned that she was a different creature from a different world, and it was fortunate that Spike was there to point out to her that it was what was in her heart that was important.
There is this moment:
http://www.youtube.com/watch?v=qj8mG_YQCjk
She does hug him and tell him that no one could ever replace him, both of which are more or less expressions of love (Also, she and Owlowiscious just rescued him)