A theme that follows all the best practices to learn from?

Are there any good themes that follows all the best practices, to the letter, that I can refer to will creating my themes?

WordPress’s Twenty Ten and Twenty Eleven (which both ship with Core as of this writing) are most “official” themes. There’s also the recently released “underscores” or “_s” theme from Automattic. Twenty Twelve will come out with WordPress 3.5 and is made by The Theme Foundry, which I think quite highly of.

There are lots and lots of good themes, but I’d recommend starting with the ones written by the people who make WordPress.

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EDIT: If you want to modify Twenty Ten or Twenty Eleven, make sure to do it using a child theme so that your changes aren’t lost when those themes receive updates. However, _s is intended to be a starter theme and NOT a child theme, so for that one, take all the files and turn them into your own theme.

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EDIT II: And of course, now there’s Twenty Twelve.