The one in my sig about the butterfly is my favorite.
There's also a lot of others. I can't remember what book it's from, but I learned a lot of terminology for barfing from Marco:
"... You ever notice how many different ways there are to say 'throwing up'? There's vomiting, of course. Hurling. Tossing your cookies. Puking, a classic. Ralphing. There's cascading. But I prefer the terms that are more real. Like blowing chunks. Spewing your guts. Tangoing with the toilet. That's a good one. Technicolor yawn."
There's a lot of good humor, especially in the earlier books, that went over my head as a kid first reading these books. It's why Animorphs remains such a great series even now.
As for my favorite pairing...
Tobias shrugged his human shoulders. He held up his hands to stare at his own fingers. "It is me," he said, sounding like he doubted it. "My old body. Here."
I ran to him. I don't really know why, I just did. I wanted to touch him. To know he was real.
"Ah!Ah!Ah!" he yelled. He jumped back and suddenly threw his arms up and down. He was flapping, trying to get away. Trying to fly. I had scared him by rushing at him. "Sorry," he whispered, terribly embarrassed. "Sorry."
I put my arms around him and hugged him tightly.