Mac: The dictionary in OS X is one of our favourite unsung gems, but it's certainly not perfect. If you're unhappy with the results you get, Macgasm points out that they're easy to customise.
Just open up the Dictionary app, then click on Dictionary > Preferences. Here, you can reorder the search results and remove dictionaries entirely. For whatever reason, the order you pick doesn't seem to translate to the hotkey (highlight a word and tap Command+Ctrl+D), but the dictionaries you decide to include or exclude do.