1. Amok Time -- because it's all about Vulcan sexuality! Hot blood, green blood, it's all boiling when Spock goes into Pon Farr and must return to Vulcan to take a mate. This was the 2nd season opener and can you imagine what it must have been like to have seen this for the first time back then? Completely awesome and featuring one of the most famous musical riffs ever to come out of TV!
2. Journey to Babel -- because we get to meet Spock's human mother Amanda and his Vulcan father Sarek, played to perfection by Jane Wyatt and Mark Lenard. The Enterprise is in a tizzy when a shipload of intergalactic ambassadors is aboard on their way to a meeting, and Spock's father's sudden illness forces the First Officer to assert priorities which pitch his Human side against his Vulcan identity. From the series' 2nd season.
3. The Naked Time -- because we see Spock breaking down when an alien virus infects the crew and hidden emotions come to the surface. We definitely learn here that Nurse Chapel has a thing for Spock, too -- and can you blame her? From the 1st season.
4. This Side of Paradise -- because we get to meet Spock's former ladyfriend and watch alien spores loosen his Vulcan reserve. The lovely Jill Ireland guests as Leila, and Spock gets the chance to cavort around the countryside and also have a bare knuckle fight with Captain Kirk! 1st season.
5. Return to Tomorrow -- because Spock lends his body to a lecherous and devious alien entity and it's fun seeing him be sort of sexy/evil. Great guest role for Diana Muldaur, too.
6. Mirror, Mirror -- because we really like Spock being sexy/evil, especially when we're in an alternate universe and he gets to wear a stylish goatee. Great acting opportunity for all the cast in this one and many great Spock scenes. 2nd season.
7. The Enterprise Incident -- because Spock gets to go undercover and seduce an alluring female Romulan commander played by Joanne Linville. This is also the one where Kirk gets pointed ears. 3rd season.
8. Plato's Stepchildren -- because this is such a nutty episode and we get to watch Spock singing, dancing, crying and rueing it all afterwards. One of the better episodes from the very uneven 3rd season, with a great guest star in the brilliant Michael Dunn. We're not saying this isn't silly bordering on the out-and-out bizarre, but lots of interesting Spock in it!
9. All Our Yesterdays -- because Spock goes back in time and falls for the exiled beauty Zarabeth played by Mariette Hartley. Who doesn't love Time Travel, and it's always fascinating to watch Spock lose control. 3rd season, and the 2nd to the last episode of the show filmed.
10. The Menagerie, Parts 1 & 2 -- because Spock is the main engine for the action here and we also get to see the earlier version of his character. Spock kidnaps his former captain, now horribly mutilated, hijacks the Enterprise and heads for the most forbidden part of the galaxy. Very science fiction-ish, serious and interesting. From the 1st season.
Happy Birthday to Leonard Nimoy!