Six Flags Great America was probably the most disappointing park I've visited. It's not so much the physical park (which is mostly nice, though it is lacking a truly standout steel coaster) but more the way it is run. In 2014, I visited for an evening and the entire following day, and due to horrible operations that still was not enough time to do everything. Regular lines were 45-90 minutes for EVERY coaster (with most being 60-75 minutes), and even with a Flash Pass I still had to wait 30-45 minutes at each attraction due to dispatch times. The worst offender was Viper, which averaged one train every 7 minutes, but Demon, Raging Bull, Whizzer, and X-Flight also had excessively long (4+ minute) dispatch times. To be fair, I did manage to get on all the major coasters with a couple re-rides and I did leave the park for dinner (though that was because the line for lunch took 30 minutes with only two parties in front of me), but I had very little time to check out any non-coaster rides and did miss the non-Flash Pass coasters (except American Eagle, which I was able to skip the line for via an exit pass due to getting stuck on Demon for 30 minutes). Perhaps I hit the park on a bad day, or perhaps it has improved since then, but until they add a notable new coaster I'm not likely to plan another trip to find out.
For most overrated park, I'd probably say Six Flags Great Adventure. Don't get me wrong, I do like the park, but I sometimes hear people say that this park doesn't feel at all like a Six Flags park and that everything is much better here. To me, however, it felt like SFMM east with a slightly better top three, slightly better operations, and only a couple noteworthy non-coaster rides. It is a very good Six Flags park, but it is still a Six Flags park and many of the chain-wide problems are just as evident here as they are elsewhere.
Coaster Count: 535 (114 wood, 444 steel); Park Count: 110
Top 5 Wood: El Toro, Voyage, Thunderhead, Outlaw Run, Boulder Dash
Top 5 Steel: Superman the Ride, Fury 325, Millennium Force, X2, Intimidator 305