I've gotta say there are plenty of turkeys! I agree in that producing a series day and day out means that sometimes a bad one slips through the net, it's to be expected, but some really are just plain bad.
In terms of TOS the bad ones are rarely boring... simply by being TOS with that set of characters gives it a certain charm that makes them watchable. Most of the bad ones were understandably in the 3rd season and consisted mostly of "a small group of people commandeer the Enterprise for a cheap bottle show".
Were I think TOS got bad were episodes like the Cloud Minders, which have Spock boasting about his sexual prowess as I remember, a really bad uncomfortable scene!
The bad and boring episodes IMO opinion were mostly confined to Voyager, I mean The Fight, Favourite Son, Unforgetable... very bad.