What are the most effective WordPress plugins to attract the most relevant traffic and maximize website sales?

What are the most effective WordPress plugins to attract the most relevant traffic, the most relevant traffic and maximize website sales?

I am thinking maybe:
1. RSS feed plugins,
2. Cost Per Action (CPA) plugins or code snippets,
3. Affiliate plugins and/or links

Can you provide links to the tools that you think are the best, and let me know why you think they are the best?