All of them- what reason do I have to suspect you are lying to me?
At least you are asking a fundamental question.
In addition to the question you are asking, though, you should ask "What reason do I have to suspect you are wrong?" and "What reason do I have to suspect you are exagerating?"
Does my accepting your experiences might differ from my own force me to invalidate all my own experiences?
OK, let me jump into more obvious examples, getting closer to a less obvious example:
1. I claim that, when playing D&D with you as the DM, I rolled a natural "20" on every die roll I make. However, I do not wish you to look at the die; you should simply take my word for it. It is my experience, in the game, that I am rolling these "20"s, and I expect you to believe it is so because I say that is my experience.
2. You DM, sequentially, for several hundred persons over a wide geographic area. In many of these cases, players stated a preference for foo, but when you introduced foo to the game, in exactly the manner they stated a preference for, in each and every case, the game ended with everyone unhappy specifically because of the foo. Each of these players expresses disgust with the effects of foo on the game, and leaves, never to return. Now, another group of players arrives, and they also express a preference for foo. Do you give them the same credence you gave the hundreds of previous players, or have you learned from your experiences? If you accept that they are telling you the truth, how does that relate to your previous experience? If they also leave in disgust, what about the next group of players who say they love foo? What about the next? The next? At what point do you stop assuming that the next group of "foo loving" players will know what they are talking about?
3. A poster gets into a long and complicated discussion on EN World, claiming repeatedly that he believes fudging is a bad idea that damages the game in nearly every case. Now he asks you if his statement of "In my experience, and IMHO, fudging is beneficial to the game" should be given the same credence as his statement of "In my experience, some players like to play human characters, and some do not." Do you conclude that these statements have equal claim to veracity? Or do you maybe....just maybe....learn from prior experience?
RC