I love the first two series narrated by Ringo Starr, produced in 1984 and 1986. Series Three, which contained multiple stories written by Britt Allcroft and David Mitton and not adapted from the Rev. W. Awdry's Railway Series books, was pretty boring. Series 4 and 5 got a bit better, but still didn't live up to the standard set series 1 and 2. Series 6 re-hashed old material, and I'm sorry to say the show has been on a decline since then.
The stories that are being written today are completely senseless. Lots of things happen that either wouldn't (ie. road vechiles have right of way over trains) or are impossible (ie. successfully making it snow by singing songs and being useful.) Awdry's stories, despite the fact that they were about anthropomorphic trains, were at least realistic.
I was born in 1995, and I prefer the 80s episodes to the 90s ones.