The Watchmen set standards that are indicative of a phase change in comics. It's no longer a medium for kiddies-and Watchmen was one of the few pieces to raise the bar. Others may have written better(not IMO) but this set the standard, along with Frank Miller's DK Returns and that amazing run of Swamp Thing. Preacher is cool but came out way later. Sandman is cool. Watchmen just looked at things differently and when it came out no one was actually doing that. Hence the reaction to it over the years.