Although I believe that Tony wasn't killed in the final episode, for purposes of discussion,let's say he was.
My prime suspect for ordering the hit is Butch DeConcini.
Even though he played nice at the sitdown in which the OK to whack Phil was given,I never trusted the little weasel.
After Phil's death, I'm pretty sure that he was made new Boss of the Lupertazzi's.
I think he figured that with Tony gone,and nobody really left but Paulie,he could easily absorb all of the Soprano's businesses and effectively end that Family.
Paulie was a true, tough street guy, but way too emotional (and nuts) to be a Boss.
As far as Tony getting whacked in front of his family,it may have been payback for Phil getting it in front of his wife and grandkids.
Just my 2 cents worth. Whatta you guys (and gals) think?