I've found that a couple of the hardest to get are Akershus PPO breakfast and Bon Voyage breakfast at a "normal" breakfast time. I saw several BV this year for like 11:30 but none for 8-9am, even 10am. Last year, I used touring plans' reservation finder for it and it took 3 notifications with me clicking on the link literally as soon as I got it before I was able to get one. We did it last year and it was a GREAT experience(easily the best character interaction due to Flynn Rider)but I wasn't going to go nuts trying to get it again this year. Akershus PPO is another one that you can get using the reservation finder but normally won't just come across even at 180 days out at 6am.
CRT is easily the hardest to get in my opinion, regardless of the time you're looking for.
Trouble getting 'Ohana has always kind of befuddled me because there's always multiple options when we look for it.
BOG breakfast is a great value on the Dining Plan but not sure I'd want to pay $30 OOP for a plate of eggs, bacon and toast. The ambiance is cool and all but it's too expensive for a non-character meal that has just ok food in my opinion. I guess when it first opened it was more of an "upscale" place but don't necessarily agree with that now, at least for breakfast. As far as getting reservations for it, I've had success without using the reservation finder but you have to get on quick at 180 days out. I've never even tried for dinner because the menu doesn't really appeal to us.