As the question implies you can only choose 1 tag!
I would like to defend the <title> tag as better than the <a> because you have very little control on a website over the <a> tag apart from the navigation. It is there for left down to marketing to promote the website outside a SEO role and scope. There for I say the <title> tag is one tag I could not live without if I was doing SEO.
I would say: never say that again!
And a question for candid_kris: Alt Tags (Use descriptive text with your keywords) - how exactly is that important for SEO and why?