Google

Google will send you the most qualified traffic so concentrate the majority of your SEO efforts on Google. Don't ignore Yahoo! or MSN but Google is king of search so give it the respect it deserves. With its new browser, Google's influence will only grow stronger so you must optimize your pages for Google. Use Google's Webmaster Tools and Google Analytics to fine-tune your pages/content for Google. I also use Google Alerts to keep up on my niche keywords and for comment link-building on the newly created pages Google is indexing.