Magnum Mania!

I've heard that somewhere around the middle of the show the ratings began to slip and that's why darker/edgier elements were slowly brought into the show with a more serious tone. Is this true? If so, what year did it start?

Also, which seasons are generally considered to be the best, and which are generally considered to be the worst?

I think you may be referring to the shift in tone marked by Season 6s Death and Taxes (Fall 1986), which borrows heavily from the vibe that Miami Vice created with much success. Personally, I think this was an update to stay current - much in the way the new show theme brought the show firmly into the '80s. It's interesting to note that this was the first episode that Selleck produced, which suggests the big guy was particularly influential in the shift. Thankfully, however, there was still a lot of fun episodes and the show never went down the dark (but entertaining) path blazed by MV. I'm not certain about the ratings being the driving factor.

With regard to the strongest and weakest episodes, you should have a read of this thread. I think you'll find the first few seasons rank consistently higher than the others - but not for everyone!

It actually started in 84-85, when the Cosby show started airing opposite Magnum. 82-83, Magnum was tied for 3rd in the top 30. 83-84, they were 6th. 84-85, they dropped to 15th, with Cosby at 3rd! After that, Magnum was not in the top 30 again!