Anytime Yuuri wakes up to find Wolfram in his bed.
Special mention should go to the first time this happens at the end of Episode 7. Yuuri wakes up to find Wolfram in his bed. Naked.
"Oh, I forgot about that." "You forgot?!" "Well, it's not a big deal. Pretend it never happened." "I can't do that! It would hurt my self-respect!" "Well, you can reject me, then. I don't care about my pride." "I can't do that either!" "Why not? Hey, Wolfram, wait! Do anything but shut yourself in the closet!" It's actually a wardrobe.
Everytime Gunter suddenly begins Fanboying over Yuuri. Even funnier when he does it with a completely straight face.
Sometimes while Yuuri is away on his adventures, Anissina will make Gunter and Gwendal test her rather strange, and sometimes downright ludicrous inventions. The usually straight faced Gwendals reaction during this time is what really makes these moment funny.
In episode 2, Gunter gets a nosebleed after giving Yuuri this worlds standard set of underwear. The smile on his face doesn't help.
Wolfram "heals" Geigenhuber by yelling at him.
Nicola going into labor prompting most of the main cast to begin freaking out. The fact that the episode started out so seriously only makes it better.
Yuuri's breathing method for rowing a boat.
Wolfram gets sick nearly every time he's on a boat and its usually played for laughs.
The Beauty Pageant from the second OVA.
The winner was Yozak if that gives you an idea of how ridiculous it was.
Gunter accidentally developing a very very successful modeling career on Earth and then getting swarmed by women at an airport.
Shori's first experience in the demon world can be very funny.
From getting licked by horses, finding out his 15 year old brother has a 10 year old daughter, getting hit on my Lady Celi, and almost being used as an experiment by Anissina, he probably had a very fulfilling experience.
At the very end of the second season most of the main casts pretty much accepts that Yuuri is gone for good. So the looks on all of their faces when he suddenly pops out of the fountain unannounced is priceless.
Along with that is Muratas admission that even he didn't know if pushing Yuuri into the lake would work.