Left is best.
Bottom: You lose valuable scrolling real estate, especially sinc apps respect the dock's space.
Right: 2 probs. First, the dock will constantly interfere with clicking on the scrollbar which for some reason must be on the right side. Second, downloads will end up under the dock and generating visual noise.

I keep mine pinned to the upper left and transparent, it's a good placement.


-- Gnarlie's Applescript page:
http://Gnarlodious.com/Apple/AppleScript/

If mouse navigation to/from it seems awkward, left isn't necessarily the best position when you frequently interact with the Dock on a large display. But a longer row of larger Dock icons along the bottom of a large display might also be awkward. And on the right it can interfere more with default icon placement, as mentioned.

I think bottom-center is a sensible default but obviously there are valid arguments for all the other possible permutations based on display type, application usage, and whatever else that contributes to someone's choice. One point of general consensus might be that Apple could provide the option to completely disable the Dock for folks who'd prefer not using it at all.