nolan was just plain bad. guy knew a fair amount, but he was a total politician and couldn't fix jack. and he was an a*****e who'd throw anyone under the bus to save his ass.
singletary, abosolute trash. he had no business being a head coach in the first place. for everything you can fault his two predecessors for, they could at least remember important things that happened in the game, even if they followed it up with lame excuses. singletary just plain had no clue. if he didn't have the benefit of a squad with years of high draft picks stacked up on it, he never would have won anything.
so if i have to rank them, singletard was the worst, followed by nolan-erickson in a tie.

...Even though the team was in salary cap hell, and Ken Dorsey & Tim Rattay were our QB's , he never put a strong team on the field, his debut as a coach in week #1 was decent, but the team could never get it right the rest of the season ...what a horrible season that was
Nolan now gets my vote for worst HC in the history of the NFL!!! not just the 49ers!!!