Countries attacked by NATO since 1991: Serbia & Montenegro, Afghanistan, Iraq, Pakistan, Libya
Countries attacked by Russia since 1991: Ukraine and Georgia.
Interesting.
Correction
Countries attacked by NATO in its entire history: Afghanistan.
The question here is whether NATO is imperialist, not whether its member states could be called imperialist. NATO article 5 have invoked once. As such NATO has only waged war once and that was after unprovoked attack on one of its members.
So based on the military history of NATO, it's not an imperialist organization.