I was so certain I had already posted this yesterday but anyway
While it is not exactly the conventional kind as one would consider, it was a perfectly-staged betrayal nonetheless. And thus L (Death Note) was betrayed by another L;
The only other thing that would ever top that is the betrayal of oneself. But since I cannot think of something that would fit into that category at the moment, this, instead, is the most poignant and profound of the fairly recent ones in my memory.
Gintama episode 130: "Cat-lovers and dog-lovers never keep peace"
If in one's death comes betrayal, then there was much of it in this episode; though even I would argue with myself about it every now and then.