I want to second Al's comment, with a specific reference to a single game. Major thanks to baseball-reference.com and Project Retrosheet, from which I was able to confirm my recollection.

On the Sunday evening of July 7, 1991, the Cardinals and the Cubs were on ESPN. The Cubs had brought up Laddie Renfroe for his cup of coffee as a reliever. He was a soft-tosser whose major-league career consisted of four games. July 7th was either his second or third of them. We went into extra innings against the Cards. This was the last game before the All-Star break. Only two other relievers were had been used that night, and only one had been used the previous night, and for one inning. Renfroe came in in the bottom of the 9th with 2 outs, blew a save and gave up two hits (run charged to McElroy), but he got through the 10th and 11th unscathed. The Cubs got a run in the top of the 12th (Dascenzo doubled in Dunston.)

Now, what do you do? You have a 29 year old reliever in line for his first major league victory. He's already pitched 2 1/3 innings and faced 9 batters. You have a rested bullpen, plus no pitcher who will be used in the next 3 days. The heart of the Cardinal lineup is scheduled (Zeile, Guerrero, Jose). What do you do?

That's right, you leave him in to be pummeled for 4 hits and one intentional walk in 5 batters in the bottom of the 12th. What a brilliant manager.

I was also going to nominate Gene Michael, but to be fair, he took over a sinking ship in 1986 that John McGraw or Connie Mack would not have been able to save.  In 1987, though, he had Sandberg, Dawson, and a rejuvinated Rick Sutcliffe.  The problem with that team was that the remaining guys from '84 (Moreland, Durham, Davis, Matthews, and Dernier) were getting older, and although the farm system was producing some great players, only Greg Maddux had made it to Chicago at that point.  

You make an interesting point about Durocher, though.  I guess a guy with great talent to work with who does not win deserves his fair share of scrutiny.  OK, you could chalk up 1969 to the Mets just getting hot and going on a Cinderella run.  But in 1970, they had the same team plus Joe Pepitone, and they still did not win.  

While he was here he had no concept of how to handle young pitching or pitching at all considering that every year he was here Cubs pitching led the league in pitcher abuse points. That abuse might have been worth it if it produced a WS but it didn't.......he rode Prior and Wood all year like rented mules even in blowout situations when they had little practical purpose for even being on the mound and then when it really mattered......like in the NLCS......instead of using SP in relief he went with guys like Dave Veres? Mind you, guys he was reluctant to use during the regular season. Clement might not have wanted to pitch and Zambrano might have shitty at the tail end of the season but if your going to lose do it using your best guys, not scrubs that you wouldn't even use in 8-0 blowouts during the regular season. That was unforgavable imo. Add to that......

-2004
-The Neifi experience
-Freddy Bynum
-His ego
-Latroy as closer

and numerous other head scratching moves and at least from my experience watching the Cubs (1990-1997) and later from (2003-20??) Dusty without question was the worst manager I've ever seen. I think Hendry plays a small part in it too.....he signed Baker when he should have gone with Freddi Gonzazlez and worse yet after 2004 he kept Baker for another 2 years.

...one word.

...suuuuuuuuuuuuuuuuucked.

According to some accounts, Frankie Frisch actually tried to fine his players for talking with opposing players, which I'm sure is a philosophy straight from his Gashouse Gang Cardinals teams of the 1930's.  But let's face it, the Cubs of the 1950's were not going to win anything, so what difference did it make if his players actually spoke with guys wearing a different uniform?  I think Frankie was out of the game pretty quickly after his Cubs stint was over.