What is the optimal way to display a multiple choice list?

I'm looking for suggestions for displaying multiple choice lists.

My first (and naive) solution. Obviously, 90° vertical text is unnecessarily hard to read and takes up a lot of vertical screen real estate.

Slanted text works better. Takes up less vertical space and is easier to read.

Adding lines might increase usability, although IMO it's less aesthetically pleasing.

This solution is quite different from the others, but might work well for a limited number of choices. One thing I don't really like is that this layout seems to communicate that it's almost mandatory to select at least one option, which is not the case.

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The question

Are there alternative solutions that, for example, provide better readability and take up less space?

The choices need to be visible at all time. The horizontal space is limited (around 640px). There could be as much as 15 checkboxes.

I'm not against going in a completely different direction, but I can't think of anything that would be superior.