The second line is best!
Why?

Rapid movement of the eye over "difficult" is facilitated by the "ffi" ligature, while the suppression of the "ff" ligature in "shelfful" assists the eye in discerning a semantic break. (Admittedly, this is the tiniest of semantic breaks--"shelfful" is not synonymous with "shelf full." After the earthquake, the professor's study contained a shelfful of books, but had no shelf full. And you will now very likely have noticed that ligatures, while available in TeX, are absent from the HTML browser font used in this present paragraph.)

Back to the two-minute TeX test.