I had to think for a few days to get my answer

I have owned my Star Trek and Playboy for 20+ years, so I suppose I am a bit meh over that group

Admittedly, during my visit to Daniels earlier this year, the only game I HAD to play was Strikes and Spares

But I would choose group 3, when pinballs started to talk.
Not only was sound improved beyond bips and bops, game features leapt forward from the EM days
Timed drop targets (Black Knight)(Williams, I know)
Self dropping drop targets (Xenon)
Awareness of where the ball was, (the saucer in Flash Gordon)
And general lighting effects

(I am sure there is more, but my knowledge of these games is limited)

I think that pinball machines from this era showed what pinball machines were to be in the coming years