The classic Yoshi sound...
"Fweet-fwoo!"
And, of course...
"Yoshi!"
"wowowowowowowwow!" (falling)
"Briiiiiiiing-HA!" (ground pound)
"Whooooooooaaaa!" (throwing an egg)
"Hrrrrrgghhhhh!" (flutter jump)
"Wow!" (after eating a flower)
"Ba-dum, ba-dum, ba-dum" (standing still)
"Yummy yummy yahoo!" (after eating something he REALLY likes)
Okay, I'll stop now. ^_^