I'm a noob but also an Eagle teaching future Eagles so I'll write down a few experiences:
Environmental Science is hands-down my least-favorite. No matter how fun you try to make the planning phases of experiments and observations, the Scouts think it is simply too much like school. I remember being in their shoes a decade ago and I can sympathize. No, the merit badge book is not horrible nor are the requirements (old and updated). The only thing I absolutely hope my Scouts get out of it aside from the common sense knowledge comes from the timeline requirement where the BSA's roots and relation with early 20th century conservation is revealed to be part of something quite epic. Because of this I can understand why it is still Eagle-required, just as Bird Study was in my grampa's day.
Personal Fitness is my second least favorite as I am the "bad MBC" who ignores the sit and reach box and simply uses a level tape measure with several spotters. Just kidding, this one is actually quite fun to teach and has great results with those who work to finish the badge.
Community has actually been one of my favorites to teach because we got quite a few laughs at the local County Commissioner's Meeting.
While Community is a harder Citizenship badge, it can be quite fun if you stay organized and plan well. Maybe it's not kosher (or courteous or kind) to have a few after-the-meeting laughs at the demonstrated headaches of local government but the Scouts were really learning about the process.
Family Life is one where I fall in the middle. I did NOT like taking this one as a Scout. Looking back, the 3 months of chore records was not unbearable. Though it has this make-or-break requirement, it has a lot of other stuff to offer. Recently, they added the "what makes a good father requirement." Also, this badge may in fact be the only time other than the Eagle Board of Review and occasional Scout Spirit meetings where the core character behind a Scout is discussed. Though Scouts usually like it about as much as Env. Sci., I honestly think the method and intention of the badge is spot-on. If a Scout can't discuss matters of character and morals with his family/guardians then who can he go to?
Personal Management is one I've not taught to multiple Scouts but the guy who did recently did an excellent job. He taught it on a chart through a logical vocabulary of $-concepts that kept building and reinforcing. At first I thought he was teaching it really slow but I actually learned a thing or two in the process. Personal Management, contrary to some opinions, can be taught well but it can't be rushed either or the concepts in the merit badge book will go in one ear and out the other. (I'm in the middle on this one as well.)
End of my rambling.