Shawn. Let me explain my opinion.
Dexter has an Olympia, so does Dickerson. Does that mean at his peak he was better? Not at all. Dexter has more wins than Shawn, so does Branch. Does that mean at his peak he was better? Absolutely not.
Shaun was obsessed with aesthetics and held true to the bodybuilding ideal, Dex went for size. He has no calves and had a very noticeable difference between his arm sizes. Sure hes bigger, and yes he has comparable conditioning (prime for prime not as good IMO) but Shawn had a more complete physique that was better balanced. Everyone bitches and moans that "size ruined bodybuilding" but its the same people that are the first to say, "...but X is bigger".
The era's say it all, when Dex was competing in Shawn's era he called him out saying he was gonna kick his ass. Never happened. Dex was getting top 3 in GP shows when Shawn was competing EXCLUSIVELY in the O. How many wins would he have had if he decided to do the tour and compete 4-5 times per year? How many Arnold's? I have no idea, but i can only guess it would be ALOT more than he finished his career with.
I like Dex, i think his physique is exceptional but i think Shawn has him at his very best. Just my opinion.