TOS is my favorite. It wasn't a franchise then, and so could afford to take risks that the later Treks couldn't. TOS was edgy, for its time. Can't say that about any of the later Treks.

Of the new Treks, Voyager is my favorite. I know it's the red-headed stepchild of Treks, but to me, VOY came closest to capturing the warmth and chemistry of TOS.

Until it became the Seven of Double-D show, anyway...