The only one I was aware of was the bomb attempt that was foiled at the last minute when someone at the table moved the bomb-containing briefcase to the wrong side of the oak table leg.
That was July 20, 1944, Count (Graf) Claus Shenk von Staufenberg. He lost an eye and hand in Africa, yet was the only one with access to the briefing room who would dare to make the attempt. He was also head of the coup back in Berlin. There were generals with divisions ready to support the coup, but it started falling apart since Hitler survived.
The best chance for a coup was in 1938. The order to march into Czechoslovakia was to be the cover for the coup. Clever. However, Hitler and Chamberlain reached an agreement. Over time, Hitler put more and more of his guys into key positions. But it was basically the same idea used in July 1944.
The plotters were shocked that the bomb disguised as a gift of Cointreau did not go off. It was not easy to get a bomb onto Hitler's personal aircraft, remember . . . It had an altitude detonator as I recall. Someone had to retreive it from the intended recipient!
One young officer recruited by the resistance volunteered to have a bomb on his person, and was planning to hug Hitler during a review of uniforms and equipment. Problem is, Hitler did not get close enough to the officer . . . This was not an easy decision for a young officer to make, not believing in suicide but having to come up with some way to assassinate Hitler . . .
I disagree that the attempts were poorly planned. These men were serious military planners, and personally heroic. Von Staufenberg was shot on July 20. About 5,000 were arrested in the following sweep (including his wife, who had their last child in prison). Some of those Hitler had killed in very gruesome ways were made known to him by the Brits. They had information from the resistance as to who would be in the new government after the coup, and announced it on the radio. The only good German is a dead one, remember . . . Some of these people were listed to be approached for various departments in the new government, but were not in on the plot, which had to be kept as closely held as possible for security reasons . . .
Wm. Casey, Reagan's CIA director, had been in special services in WW II and concluded that even the Normandy invasions would not have been necessary, if the Allies had used the available opportunities to work with the German resistance. These were not dictators in waiting, but some really good men who wanted a decent government for Germany. Of course hindsight is 20/20, but a lot of people died because the resistance was not successful in taking out Hitler . . .