"Journey to Babel" is deservedly one of the all-time beloved episodes of the STAR TREK fan base. This is probably because this is a Spock-centric story (always popular with the Trekkers) that reveals important back-story about Spock's family and childhood. We also get to see Spock make "logical" choices that would make any human blanch, most notably his refusing to give his father a bloood transfusion because Starfleet regulations say that he cannot relinquish command of the Enterprise for personal reasons.
It's the winning performances of Leonard Nimoy, Mark Lenard and Jane Wyatt that carry this episode through - though the political intrigue sub-plots don't hurt a bit, either.
One of the best of the second season.