If you ask yourself, "What’s the best thing that happened today?" it actually forces a certain kind of cheerful retrospection that pulls up from the recent past things to write about that you wouldn’t otherwise think about.
If you ask yourself, "What happened today?" it’s very likely that you’re going to remember the worst thing, because you’ve had to deal with it—you’ve had to rush somewhere or somebody said something mean to you—that’s what you’re going to remember.
But if you ask what the best thing is, it’s going to be some particular slant of light, or some wonderful expression somebody had, or some particularly delicious salad. I mean, you never know...