The pigs, most notably Napoleon and Snowball, are very persuasive and can utilize propaganda and manipulation to achieve their personal goals; in addition, they seem to take advantage of the fact that the other animals are ignorant and highly dependent on authority. I'm not sure if the pigs are really "good" leaders in the sense that they are moral and just; I mean, people can argue that Hitler was a "good" leader because he basically led an entire country to buy into his vision of a perfect society. Perhaps a different adjective would be more accurate...?
Napoleon seems to be the true leader of Animal Farm - he's the one who gives the inspirational speeches, rallies the animals to revolt in support of "Animalism", and creates the illusion that Snowball is causing many of their problems. He also uses Squealer as his "propaganda machine" to mislead the other animals into believing that Napoleon is always right.
A good leader, in essence, motivates others, creates a vision that others want to follow, and becomes a role model for others; if the pigs (or Snowball) seem to fit this description, then I believe they can be considered "good" leaders. These qualities can be the basis for your speech - simply elaborate on them and provide specific examples from the story to support each of these traits.
I've included some enotes links that can provide further evidence and support from the story.