When Harve Bennett was preparing for The Wrath of Khan, he watched all 79 TOS episodes. He said about 1/3 were excellent, 1/3 were okay and 1/3 were stinkers.
I think Bennett is a genius but I have never agreed with this statement. Going with 78 to make the math easy (78/3), 26 just okay and 26 stinkers? Nope. I don't think so.
Gotta agree with Phaser Two on this one. It's not even close in my opinion. Most of TOS was very engaging, entertaining and high-quality. I think the S3 "bashing" is very group-think-y and en vogue, but before I started reading that on the inter webs, I never differentiated S3 from any of the other 2 seasons. Each season has great episodes, and each has small littering of sub-standard stuff. I'm not sure I'd call anything a "stinker..." as I find there are interesting and redeeming things in even my least-favorite episodes.
I also don't think it's fair to blame Freddy F. He did what he had to do with a limited budget, less Roddenberry involvement, etc.
Honestly, I kind of like the look and tone of S3 the best. It has a really weird and other-worldly feel that I really like
S1 gets a lot of credit, but there are some substandard episodes in there (Mudd's Women, Alternative Factor) as there are in S2 (Who Mourns for Adonais, Catspaw, Bread and Circuses)...not to mention S2's proclivity for leaning on the "parallel Earth" stories.
S3 has some of my favorite episodes:
Enterprise Incident
Paradise Syndrome
In Truth No Beauty
Specter of the Gun
Day of the Dove
Lights of Zetar
All Our Yesterdays
And...26 "stinkers???" No frigging way. I'm sorry...but like I said, it's not even close.
The only episodes in the entire catalogue that I find
sub-standard are:
Mudd's Women
Alternative Factor
Who Mourns?
Catspaw
And the Children Shall Lead
Plato's Stepchildren
Turnabout Intruder