Arnold, Lee, Dorian or Ronnie. With different arguments you could make a case for anyone of them being the best. Is Arnold best because he won six straight and came 5 years later and still won? Or is it Lee Haney winning eight and his last win being really impressive against tough line-up? Dorian's six straight with toughest overall competition and nearly perfect pro record? Or Ronnie being involved in two the most toughest Olympias(98, 99) and raising the bar physique wise?
I would rank them(by criteria being dominance, physique and level of competition).
1. Ronnie
2. Dorian
3. Arnold
4. Lee