Many people consider 50onRed (under International Web Services LLC: http://www.advertising-support.com/) as the biggest company in the field. According to a publication from March 2014, they serve up to 1.5 billion impressions per day (http://adage.com/article/digital/big-ad-injection-scheme-hit...).

I'm not sure, but if adwarekiller's numbers are correct that could put them up there.