The acting in TOS was very melodramatic a type of acting that has gone out of style. I think there are some good episodes that will last with TOS and some that are very dated. Since I grew up with them they are guilty pleasures.
I mean you won't see episodes like "Plato's Stepchildren
" "Spock's Brain
" or "The Gamesters of Triskelion
" made into a feature film. They are a bit silly.
Those are episodes I've usually find entertaining because they're so silly you can't take them seriously. Also the ones that use things that have now become sci-fi clichés like defeating computers with a logical paradox and defeating enemies by seducing them or teaching them about love. Even in the best episodes though there are often moments that just make be laugh out loud with unintentional silliness.