Final answer:
Transnational bargaining has been the most successful method for unions to negotiate better conditions and leverage collective power against MNCs, addressing their transnational nature.
Step-by-step explanation:
In addressing the question of which method unions have been most successful in implementing in their dealings with MNCs (multinational corporations), it appears that transnational bargaining has been a significant tool. Given the global economic integration and rise of trading blocs, MNCs have grown in power and influence. However, despite their positive contributions to economic growth, MNCs have been criticized for offering suboptimal working conditions and for being a driving force in the homogenization of cultures, which critics liken to a form of neocolonialism.
Workers in many areas have been prevented from forming unions and are often required to work long hours in unsafe environments. Consequently, unions have found transnational bargaining to be an effective method to negotiate better conditions, as it goes beyond local organizing and addresses the transnational nature of MNC operations. This method allows unions to leverage a collective approach across borders, juxtaposed against other methodologies like boycotts, strikes, or codes of conduct, which might not be as uniformly effective given the divergent political and regulatory environments in which MNCs operate.