Third-Season Man From UNCLE

Third-season Man From UNCLE...

I don't remember the sequence/timing of deterioration, but I remember it getting funkier. The gadgets and campiness were supposed to be adjunct to a real spy drama, but took center stage. (At least this gave us Kirby's Agents of SHIELD comics.)

Batman was best in its first season (again IIRC) - they had the campy biff bam pow stuff, but tried to be semi-serious (1950s comicbook) Batman. By 2nd season the biff bam pow that made it famous affected the show so it degenerated into insipid self-parody. (Ruined the early attempt to make a serious Batman in the comics, too.)

But this wasn't just a 1960s thing. Go back to the George Reeves Superman series, and there, too, you'll find the first season was semi-serious stories, Supes v gangsters, like the radio show. Then it degenerated into "Lois suspects Clark is Superman" and "Clark winking at the fourth wall" silliness.

Ah, but then, there's the Monkees, which started off as mere Beatles movie mimickry (the pre-fab four). Then the boys started insisting on doing their own songs but I don't think I watched the middle-period shows. Finally, by the final season when they realized they were being canceled, they went truly acid trip, culminating in the surreal Head movie.

I think as far as series' changing tone, the Monkees wins.

All of this IIRC from long-ago original viewing. YMMV and WTF apply.

(Washes hands to get the nostalgia stench off)