What is your preferred panel/dock/taskbar placement?

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http://www.linuxquestions.org – Windows 7 had the default (and famous) taskbar at the bottom. Others have it at the top. Ubuntu made the shift to the vertical left-sided panel. At the end it all boils down to the user preference. What in your opinion, is the most effective and preferred placement? Why is it? Is it design, accessibility or any other factor? Would love to hear your ideas! (HowTos)