The Poisoned Apple

I think that Mallory paints Queen Guinevere in the most negative light and Sir Lancelot is the most positive in The Poisoned Apple. The first example of this comes at the beginning of the story when we are told that Lancelot busies himself with other things, like doing God's work, trying to deflate rumors and save Guinevere from being dishonored. In response to his trying to help the situation, she gets angry with him and banishes him from Camelot. (Oh, the fickleness of women.)
Another example of a negative light on Guinevere, notice how quickly the Knights turn against her when they discover the poisoned apple. Not a single Knight gives her the benefit of the doubt, their trust in her must not have been very high to begin with for them to turn on her so quickly.
In contrast, when people speak of Sir Lancelot, they talk about his loyalty and good will and the fact that he can do no wrong. Even in The Day of Destiny, after everyone has learned of the adultery that went on, and after they had battled Lancelot, the Knights and even King Arthur had few bad words to say on his behalf. When Sir Gawain was dying (the wound given to him by Lancelot himself), he wrote a letter to Lancelot, begging for his return.
King Arthur, in my opinion, is painted as somewhat of a fool in this story. His Knights are all loyal, and think very highly of him, but all of this is going on right under his nose. Even when Lancelot tells Arthur of the fierce loyalty he has for the queen, Arthur goes on without a trace of suspicion.