In order of preference
Giles
Ramsay
Forss
Howson
Fry
I would say they are all very useful and there is not a big difference between the preferences.
Giles just the sheer number of assists/crosses is very useful.
Ramsay gives that extra creativity and energy
Forss - strong and direct and helps offensively and defensively
Howson - psychologically gels the team and works hard, experienced head.
Fry - reads the game well, mobile for a centre half and confident on the ball.
Of course they have have weaknesses too which I am not listing in here.
We often don't play Forss and Ramsay together because there is not enough space in the team if Akpom, Archer, McGree are playing.