Overall the third season is clearly the weakest but it's not like TOS became an entirely different show in it's last year and there are quite some memorable episodes like Savage Curtain or Let That Be Your Last Battlefield.
The 3rd season space hippies episode isn't really worse than the 1st season salt-sucker-monster-of-the-week episode so I'd claim that in TOS, like in all Trek series, it depends on the single story. The average quality differences between seasons (or even entire shows if one wants to compare the five series) is far smaller than the quality difference among single episodes.
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