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.
It is in the pigs' ability to control the truth and manipulate their subjects that makes them effective leaders. They are able to mask their consolidation of power into making others believe that they are "good leaders." Through the work of pigs like Squealer, Napoleon is able to convince the animals of his authenticity of leadership as well as how he is looking out for the animals' interest. In reality, Napoleon and the other pigs are only convinced of securing and enhancing their own power. Yet, they are able to persuade the animals that strengthening the pigs' power is aligned with the advancement of the cause of all animals in the pursuit of animalism tenets. This is effective leadership from the most pragmatic point of view because it is able to secure one's own position with the security of public support or silencing the power of dissent. In the end, I am not sure if Orwell sees this as necessarily "good," but rather a statement on how modern leadership is effective in pursuing its own agenda while being able to convince the body politic that its presence actually advances its own needs.