I did a few searches in Google books to see if I could find a pattern. It seems like more hits are found when the article is omitted rather than added. Here are my results (all searches were done "in quotes"):
Things I love most: 8310 results
Things I love the most: 1570
Things I hate most: 2060
Things I hate the most: 195
Things that bother me most: 859
Things that bother me the most: 207
Things I like best: 25,000
Things I like the best: 295
Things I do best: 6890
Things I do the best: 1870
Given that a pattern is emerging, the next question would be: Why?
I remember one tip for effective writing: eliminate extraneous words. That admonition is found all over the web. For example, such words are called flab in this blog; the same exhortation is buried into Tip #9 of this writer's guide:
9) Write more than one draft of your essay. Great writing comes from revision. Eliminate extraneous words and phrases. After you revise, be sure to proofread and spell-check your work. Proof-reading is not the same as revising!
I'm guessing that it's often omitted because it's unnecessary. Which leads me to the last pair of queries I ran (not in Google books, but just as a Google web search):
Eliminate extraneous words: 5890
Eliminate the extraneous words: 68