Me vs. Twenty Year Old Me
I am a fairly large man, and I sometimes think to myself that I would not like to meet me in a dark alley at night. This is not so much because I am dangerous, but because it would require time travel and all of the ensuing paradoxes. Meeting myself hardly seems like it is worth destroying the space-time continuum.
However, if I did meet past version of myself, I assume we would fight before the universe exploded (I do not agree with Randall Monroe that the natural reaction would be to make out with myself, although I do admit it would be kind of nice to determine exactly how good a kisser I am).
This naturally leads me to wonder if i could beat past versions of myself, say from when I was twenty. I'm actually much stronger than him, and better trained. So you would think I have a natural advantage. However, he is reckless, essentially insane, and has the ability to undermine my existence. For example, he might take up smoking in order to decrease his lung capacity. Or infect himself (that is, myself) with some sort of slow wasting disease.
So, I think I can take him, as long as he isn't expecting me.