Good Question.
I wonder out of Howe, Hull and Beliveau, who was the better defensive player? Obviously, Beliveau being a centre changes things/assignments, but I think it would be an interesting discussion.
Good question especially since Hull was a LW, Beliveau a C and Howe a RW, different responsibilities and different opponents when it comes to match-ups. So even if there was significant overlap in their careers there is very little common ground defensively.
The answer involves linemates/teammates and to a degree coaching.
Bobby Hull had the disadvantage of playing on a team with a high level of turnover of teammates and second level coaching - Pilous and Reay, a rung below Blake and Imlach. Defensively his best linemate was Chico Maki but a relatively short term combination. As a result Hull never developed the familiarity and synergy necessary to play good team defense. Add the coaching gap - Blake and Imlach tended to win line match-ups against Chicago, Detroit was a wash, Boston, NY did not matter and Hull did not have the full support that defensive play requires.
Gordie Howe and Alex Delvecchio LW/C. Played together for twenty seasons. Knew each other and the league. Together they were more effective then the sum of their individual skills. True offensively and defensively. Regardless if Delvecchio played LW or C or who the third player was they made the necessary offensive and defensive adjustments.
Jean Beliveau. Real strength was his flexibility, adapting to the various offensive / defensive systems and schemes put together by Toe Blake and the variety of wingers that he had.Played with veterans, rookies and made it work. Playing with H. Richard and Ralph Backstrom centering the other two lines the team varied the looks, switching from LHS to RHS centers, etc. Biggest asset was the ability to play against the Howe/Delvecchio combo, especially with Gilles Tremblay on LW matching against Gordie Howe. Made the difference in the 1966 SC finals.