Speaking of first to worst - I caught a rerun of Psych, season one, and was reminded how awesome the show was when it began. The first season is well written, tight, funny, and smart. Sean is likable, Gus has plenty of lines, the cop-psychic interplay is fun, and the pacing is fantastic.
By season three, the protagonist (Sean) is a smarmy asshole, Gus has basically disappeared, product placements are overwhelming the plot, and the show feels stale. Once again, the old adage holds:
In season one, the actors work for the writers
Season two, they work together
By season three, the writers work for the actors
As a case study in this truism, see the behavior of Heigl, Katherine.
Sorry Psych. It was good while it lasted.