You might look at Macbeth's soliloquy "To be thus is nothing, but to be safely thus" (Act 3, scene 1). Here Macbeth shows his opinion of Banquo as well as the reasons he had him killed. I agree with the post above, that Macbeth would hide any malice he had felt toward Banquo through his flattery of him. He would most likely praise Banquo's courage and wisdom, and mention his regret that Banquo would no longer be a participant at his castle's feasts. He might also comment on his "relief" that Fleance had escaped the hands of Banquo's assassins and perhaps somehow express his wish that Fleance be returned to Macbeth's castle where he might be "safe." Of course, Macbeth might also express the wish that Banquo's spirit rest in peace. I'm sure he would not want any more visitations from Banquo's ghost.
One important motif in the play is the idea that appearances are often deceiving, and that a "false face" covers up the truth of a person's character. As they plan Duncan's murder, Macbeth and Lady Macbeth plot to look like "the innocent flower," but be "the serpent" underneath it. They also decide they will make such a show of grief at the King's death that no one will suspect them. Since these strategies worked for them, at least for a while, no doubt Macbeth would continue this behavior.
At Banquo's funeral, Macbeth would surely express his profound (and false) grief at Banquo's demise. He would no doubt swear vengeance upon whoever killed Banquo in order to draw any attention away from himself. Macbeth's "false face" would be quite convincing during this performance. Since Lady Macbeth would surely be in attendance at the funeral, Macbeth would draw attention to her "distress," also. The two of them would put on quite a show.
I'd also add that Banquo would be more likely to imply the above suspicions, rather than state them outright.
There is little doubt that Banquo is a noble and moral man and remained so until he was murdered by his apparent best friend. He is obviously willing to take second place to Macbeth, as he doesn't resent the actual honors or the prophesied honors which he hears about himself and his friend. I commend him for not over-reacting--or, really, acting at all--when Duncan is killed. he had to have wondered, from that very first moment, whether Macbeth had anything to do with the murder. Instead, he is temperate and reserves judgment. We do have a hint that he's concerned, as he says he'd like to talk with Macbeth in private; however, he is a loyal friend for the duration of the play. Any eulogy would have to include these things; of course, there would necessarily be an undercurrent of Macbeth's distrust and paranoia, but the rest could not be ignored.
Think about their relationship...they were the best of friends, fighting valiantly side by side, and Banquo is the only other witness to the prophecy of the witches. This is exactly the reason why Macbeth has Banquo killed--not only does he have reason to suspect Macbeth for acting heinously in order to make the prophecy come true, he must also protect himself from the witches' prophecy for Banquo...that he will not be King, but he will have many Kings in his lineage.
Knowing all this, you know he would speak of Banquo's excellent character, courage, skill as a warrior and importance as a friend and companion. He may have some asides addressing to himself mostly that Banquo had to die for the reasons listed above.
This sounds like such a fun assignment! Good Luck!
I need a little more insite on an eulogy for Banquo... Please