The 1st two albums are just pure classics, every song is great. 3 and 4 were good, but for me not as good as the 1st two. After that it was just a slow march downward for the rest of their career. There were still good songs to be found here and there, but it seemed more and more forced as the years went on and they seemed to not give a crap about their later horrible albums at all.
I think a good jumping off point is when Dee Dee left the band, though they had been going downhill before that.
Even earlier, they were hurt when Tommy stopped being a producer. He got their sound and what they were all about in a way almost none of their later producers did. Even the later rather mediocre Too Tough to Die album benefited from his coming back to produce it and it helped a lot.
The thing is, at a certain point like many long running bands the guys started treating it as a job. Maybe for Johnny it was always just a job. He often said that it was. Once being in a punk rock band is a job...well that isn't really the point of a punk band is it?
My most controversial Ramones opinion: I've always thought Pleasant Dreams was a decent album. It just isn't a good RAMONES album. It's a pop album showcase for Joey. Most people seem to hate it. I like it much more than the highly overrated End of the Century. Phil Spector and the Ramones are not a good mix. Ramones songs need to be stripped down. Wall of sound style just defeats the whole purpose. For me Pleasant Dreams succeeded as a pop album where End of the Century failed as one.