I really don't care for 'worst' lists, but I can give the last 10 of my favorite 79 episodes

.
70 - Let That Be Your Last Battlefield - just too blatant
71 - The Man Trap - not what I would have picked for a premiere
72 - Spock’s Brain - actually, it's so ugly it's cute
73 - The Alternative Factor - why?
74 - Miri - kids, yechhh
75 - And The Children Shall Lead - ditto
76 - Mudd’s Women - I preferred Mudd's other appearances
77 - The Way To Eden - too '60s
78 - Plato’s Stephchildren - other than the kiss,

79 - The Omega Glory - even though it has Morgan Woodward