I'm a huge TNG fan (nostalgia goggles firmly in place) but the devolving episode has always been one where, once I remember that's the one I'm watching, I skip to the next episode. Same thing with the one where everyone got polywater poisoning and Wesley had to save the day somehow, and the one where they went to the Blaxploitation planet and the guy kidnapped Tasha to be his love slave.

The only thing that bothers me about TOS is the pacing. Everything else, like the 'lame' alien designs and visual effects are an artifact of the era; they were cutting edge effects in the 60's, but haven't aged well. Rather like going back and playing the PSX era Final Fantasies and remembering how my child-brain was blown by the FMV sequences in VII.

Even with inflation, the budget for each TOS episode was less than half of the budget for each TNG episode, and even in the better-funded TNG era the make-up and costume departments could only afford to make less than 10 aliens total to represent an entire species. Hence why the Enterprise so rarely encounters alien vessels as large as itself, why the same species show up so often, and why so many were rubber forehead aliens with uninteresting gray clothes. (Enterprise had an estimated budget of about $4 million per episode--my guess is it was a money laundering front.)