Idk what's better: EN saying MUDDLE FUDGER, shirtbag, son of a nice lady Or JP saying BABY aggressively every swear, and also my favourate line "HORI BABY!" (Or holy if you want but jp accent on english always makes giggle a little bit)
JP is closer to the original CN than EN is. He basically replaces namecalling with pet names. "Son of a nice lady" is really the most accurate thing we have in EN, since it actually changes the swear to something nice instead of just a silly sound-alike.
As a player with CN voiceover, I can say with confidence that Boothill's voice will be my favorite one for indefinite time. I laughed so hard when I idled in a fight and heard his whistle for the first time. Absolutely love it. Also it seems similar to the whistle in his ultimate, but maybe it's just me.
"Hyuuu" like he said is the japanese onomatopoeia for whistling. So instead of actually whistling, he's saying the onomatopoeia. This is pretty common in japanese media if I'm not mistaken. If you don't know what onomatopoeia is, it's kinda like saying *bonk* when you hit someone on the head with a stick or something. That *bonk* is an onomatopoeia. The sound-effects common in comics.