A eulogy for Banquo, given by just about anybody else in the play, would probably focus on Banquo's loyalty, dedication, and wisdom. However, a eulogy for Banquo given by Macbeth would end up focusing on these qualities from a different perspective.
Throughout the play, Macbeth reveals that he is terrible at being deceitful. He's a war hero; he fought valiantly when fighting for the right reasons. He can cut his way through an entire battlefield and destroy an enemy leader in one-on-one combat. However, when it comes to acting and speaking deceitfully, Macbeth has a tendency to mess up and even fall apart, even in less stressful situations. If he were giving a eulogy for Banquo, he would be the center of attention, speaking a great deal. Since he was the one who ordered Banquo's murder, he would be feeling a lot of pressure. As he shows throughout the play, speaking while under pressure doesn't tend to work out well for him. Even though Macbeth would attempt to act as if he cared about Banquo as much as the rest of Scotland, he wouldn't be able to do so.
As the play progresses, Macbeth grows more and more jealous of Banquo. Due to the Weird Sisters' prophecy that Banquo will "give birth to kings," Macbeth begins to think that all of his hard work has been for Banquo's line rather than his own. Macbeth killed Duncan to become king, but the prophecy suggests that at some point, Banquo's line will take over. Because of this, Macbeth comes to see Banquo as a thief who is stealing the kingship. Ironically, this is exactly what Macbeth did in killing Duncan. At some point, Macbeth would accidentally let slip his true feelings about Banquo.
Further, due to his guilty conscience and paranoia, Macbeth would also be likely to have a meltdown in front of everybody. After killing Duncan he nearly reveals his guilt. After Banquo's murder, he falls apart for all to see when Banquo's ghost shows up at a dinner party. If Banquo's ghost felt the need to come to a dinner party, you can bet it would be making an appearance at his funeral as well. In the former example, Macbeth appeared to be shouting at nothingness, or sometimes a stool, as he chastised the ghost for all to see. Given that nobody else could see the ghost, Macbeth appeared to have lost his mind.
In summary, Macbeth's eulogy for Banquo would probably start out by focusing on Banquo's qualities: his courage, his strength, his loyalty, etc. However, Macbeth would soon begin to let things slip, tripping over his words, focusing on his view of Banquo as treacherous, conniving, and out to get him. At some later point Macbeth would probably start to insult Banquo directly, and if Banquo's ghost appeared there would probably be a great deal of ranting and raving. Lady Macbeth would, of course, try to divert attention by swooning or knocking over a vase, and the entire funeral would end with people being even more suspicious of Macbeth and he and Lady Macbeth taking separate horses home for the night.
A eulogy for Banquo would probably emphasize that he was a loyal vassal of Duncan (and later, one could say, Macbeth) and a great warrior. These were the virtues most prized in Macbeth's time, as emphasized by the approving description of Macbeth's actions in the battle against the rebel Macdonwald. Beyond this, one could say that Banquo, though ambitious and intrigued by the witches' prophecy that his descendents would be kings, remains loyal and is certainly unwilling to compromise his basic humanity to fulfill his ambitions. Finally, a eulogy might mention the witches' prophecy itself, which positioned him as the ancestor of a number of great Scottish kings.