Either 21st TB or Piccolo Daimao. The former had a good balance between power vs technique fights, action/comedy and an ending that seems subversive of a Shonen tournament arc even today. The latter easily had the highest stakes of any arc other than the Saiyan Arc due to the lack of the Dragon Balls as plot armour, made even more apparent by the large death count of the main cast, as well as being a good tone shift for the series, though is limited by the Choshinsui being an asspull and everything returning to normal at the end (though the former is made slightly less of a contrivance anime-wise due to acquiring it being far less convenient).
I'd probably still go with the Piccolo Daimao Arc, though.