Intimidating IMO means that the guy scares opponents before he does anything just by his presence.
Boxing: Mike Tyson in his heyday. He basically made opponents wet their pants before the bell rang. Add to that the fact that he was a street criminal in his youth, a rapist in his adulthood, and by all admissions of people close to him has violent mood swings, and I think you have a genuinely scary, violent customer.
Baseball: Ty Cobb [vintage] and Nolan Ryan. Cobb won through intimidation, which is why he had so few friends. Ryan took no crap from anyone, ever. Plus his beatdown of Robin Ventura is a classic.
Football:
They Call Me Assassin was one of my favorite sports books as a kid, so I have to go with Jack Tatum. Honorable mention to Jim Brown--if he couldn't go around you, he'd just go through you. 5.22 yards per carry for his career, highest all time, and averaged over 100 yards per game for his career.
Basketball: Bill Russell. He changed entire game plans with his defensive play. 22.5 rebounds per game for a career.
Hockey: The Hammer, Dave Schultz. The baddest bad boy on the Broad Street Bullies.