For me, pretty much from the beginning to varying degrees. But if I had to nail it down, the last four seconds of season three. Not only did The Best Of Both Worlds change the way Star Trek was done, it pretty much revitalized the whole season finale cliffhanger.*
*Those eighty's who shot who night time soaps not considered for this opinion.
*Those eighty's who shot who night time soaps not considered for this opinion.