Norman was nearly impossible in my first Sapphire run, but since I always have come prepared to take him on. Still a heck of a fight. His bigger Slaking is evil - the rest of his Pokemon not so much. I just need to have a Breloom, Machoke, or Blaziken in stock.
Tate & Liza are also REALLY REALLY hard, and are probably my current answer. They're definitely hackers, lol.