Research Studios (Neville Brody)
David Carson Design
Pentagram
Dixon & Baxi
Vignelli Design
Sagmeister inc
Matsumoto inc
344 design
Meta Design
Saffron-consultants
Johnson Banks
These are a few of the top Graphic Design Firms.
Its really hard to narrow it down to one or two , because of 2 reasons: One is that, Many of these GD firms are innovative in their own way and second, It depends on personal likes.
Many Consider David Carson the father of today's new Graphic design movement.
Neville Brody a successor .
Whereas Vignelli is classical Graphic design.
Like I said its really hard to narrow it...
You could, alternately browse through the
AIGA medalist Design Archive
Or the
Red Dot award Design Archive