It's pretty funny seeing Sonic having an argument with a sword.
The look on Sonic's face when he's told he's the real King Arthur!
After the credits in Black Knight, Sonic is telling Amy about what happened. She doesn't buy it, and thinks he's just making up excuses for missing a date with her, and she's displeased.
Big's cameos in Secret Rings can be quite amusing, but the real funnies come with the diary pages you get for triggering them. Among the things he did while in the story book include attempting to fish in a desert and somehow holding a conversation with a skeleton who (according to Big) was once a colorful fish that could sing. Maybe the heat had gotten to him?
The opening scene of Sonic and the Black Knight: Sonic gets warped into Camelot, holding a pair of chili dogs, which he makes a diving save to keep from falling. After eating one, he then demonstrates his speed by tossing the other one up in the air, routing all of the Mooks in sight, and then catching it.
The latter part is really more of a Crowning Moment Of Awesome. The realCrowning Moment of Funny is when Sonic prepares to rush at Arthur... and Merlina grabs him mid-run, pulls him back, making him drop the last chili dog, and as she warps them out of the area, Sonic lets out a Big "NO!" at the loss of his lunch.
Made still funnier by the fact that Sonic's staring down the Black Knight, holding his chili dog as if it was a sword, immediately before Merlina does that.
Sonic's reaction at meeting the "Lady of the Lake" for the first time, here played by Amy. He's about to bolt until Caliburn stops him. Throughout the whole meeting, Sonic has an uneasy look on his face.
At one point during gameplay in Black Knight you have to grind some railing over lava, prompting Caliburn to comment on how easy it'd be for them to fall in.