Google citations aren't everything but if they were ...

Citations for academic staff in the Institute for Governance and Policy Analysis

Data used: Google Scholar ranking of academics by subject area. Google Scholar is the only comprehensive citation base for Social Science as it has a more intelligent search engine for book outputs. An academic is included in a subject area if they classify themselves as in that area. So, if someone who might be regarded as a Political Science or Public Policy doesn’t classify themselves as such, they are not included.

Political Science
IGPA has 3 academics in the top 20 Political Scientists in the world in terms of the number of Google citations (as of 6-2-2018):
John Dryzek, ranked 11th with 27,703
Gerry Stoker ranked 16th with 22,685
David Marsh ranked 19th with 19,032
Patrick Dunleavy is ranked 36th with 15,251

No other University in the WORLD has 3 in the top 20.

Public Policy
John Dryzek, ranked 12th
Gerry Stoker ranked 14th
David Marsh ranked 18th
Patrick Dunleavy is ranked 25th

Again no other University in the WORLD has 3 in the top 20.

Sub-Disciplines

Environmental Politics
Dryzek is ranked 2nd

Political Theory
Dryzek is ranked 4th
Dunleavy is ranked 8th

Political Sociology
Marsh is ranked 8th

Public Management
Dunleavy is ranked 10th

Deliberative Democracy
Dryzek is ranked 1st 
Simon Niemeyer is ranked 6th

Food for thought.