I played a lot of RPGA games and had many different judges, but overall I'd say only two stuck out as poor. One guy didn't know the rules very well and stunk to high-heaven and another didn't read the modules he was given nor did he bother to sleep the night before so he was running a game that he had to read at the same time while sleep-deprived. Completely boring.
In my home campaigns, I'd say three guys jump out at me as being very poor GM's. One guy loved White Wolf everything and was running a Werewolf game. I was interested, but the guy made up the story as he went along and I had to roleplay out a lot of meaningless stuff like withdrawing money from an ATM machine. One of the bigger hassles of the night was he demand that I realistically come up with a way to smuggle a gun onboard a plane (Uh? Can't I just declare at the counter and get ammunition where I end up? NO.). That killed two hours in which I was frustrated and told him, I don't take the gun. After a whole six hours of this gaming we get to one fight with some mook monster, kill it in the first round, and game over for the session. I quit playing Werewolf.
The second GM ran a 2nd AD&D game. I wanted to turn over the reins to someone else so I could get a break from running. I thought my very imaginative friend would whip up a really cool adventure for us. I was wrong as he too was making the adventure up as we went along and his adventure had absolutely no plot. No bad guys, no adventure seeds, a random encounter here and there, players haggling with blacksmith for two hours of real time on making custom full plate armor. I put up with this for about four game sessions (each was 8 hours long) before I pulled the plug and said enough.
The last guy was someone our gaming group brought it to DM our 1st ed AD&D games. He was a 1st ed AD&D only gamer which was okay with us. He actually did some prep (not much, but he at least made notes to construct a plot and provide an interesting story), but he was completely dice and rulebook aversive. If we got into a situation where it would like it would be a fight, he'd try to get us to roleplay it out instead of fighting. We would bite, but it would get old after the 10th time of running into goblins and kobolds and they want to parley. So we started attacking every monster we came across. He then had the monsters run away--even if they seemed tougher than us. We're 1st level and we're making ogres run away. Other times, he'd just call the fight in our favor before we got initiative rolled. He knew the rules, the guy was smart, but he hated using them and didn't bring any rulebooks. So if we wanted to do some action, it was either an automatic success or automatic failure. I very rarely rolled any dice in any of his sessions. All of us revolted on him after putting up with about three sessions of this and he flat out didn't care so we kicked him out.
Now I've seen my fair share of mediocre DMs and probably know only of two guys who I considered the best DMs I ever had, but all the mediocre DM's I cut them a lot of slack because they always took the time to prep and that always means a lot to me so I would meet them halfway. Some of their styles weren't necessarily my cup of tea, but their efforts in good faith to provide a fun game was far more important to me than their skill as a GM. And for that, I think makes them a great GM in their own right which makes me grateful to have played in their game.