Original's characters are better/uniquer than TNG. Let's call Kirk and Picard a draw. Spock over Data, by a mile. Bones over Crusher by a mile. She's a female "nice guy." Riker? bland, nice guy. Geordi? Bland, nice guy. Troi is a bit weird (that's good), but "nice" with horrible acting. Worf is interesting.
But, Chekov: funny, quirky, goofy Russian accent. Uhura is spunky in the films. Scotty! Sulu is still a bit bland in the movies, I'll give you that.
Overall I'd go 70-30 as far as TOS' interesting-characters advantage over TNG.
PLUS the whole Jungian (not "Freudian," O poster upstream) individuation (non-dividedness) embodied by Kirk with Spock (reason) and McCoy (passion) on either side. Very fortuitous (inevitable, I think Jung would say) that Kelley rose from day player to costar, and with him the third part of the triumvirite.