Yes, this is a really tough one for me. Those two eps are amongst my top TAS faves.
"Slaver" is cool in presenting a TOS eps without Kirk, and having Uhura and Sulu in much larger roles.
"Pirates" is just a great TOS story, love the Kirk/Spock/McCoy stuff, good adventure, a lot of classic TOS elements to me.
Tough choice. Hmmmm...
I'm going with "Pirates".