I have things I like in all of them.
Definitely, "Yesteryear" is my favourite. "Bem" comes in for a lot of criticism, but I like the quirky guest character very much and the jungle visuals are
stunning, esp. on DVD. "The Counter-clock Incident" is also rubbished by many, but I
loved the addition of the Aprils and the (weird) science blows your mind. "Albatross" and "The Jihad" are solid, colourful whodunnits.
So that would be my top five, but that leaves out "The Slaver Weapon", "The Time Trap" and "The Terratin Incident", all very successful episodes. "The Slaver Weapon" introduced me to the writings of Larry Niven. His "Tales of Known Space" and the "Ringworld" series, featuring the kzinti, are indelible in my mind. The many years, I could only watch TSW with the last three or so minutes chopped off. My grainy crosstaped copy of the episode ended just before the kzinti ship explodes!
Love the great DVD set and all the bonus features.
Chuft-Captain, M'Ress and Arex by
Therin of Andor, on Flickr
Japanese Starlog once offered the above pic as a limited edition cel. (Japan had a second wave added to the range of cels once offered by Tuttle & Bailey in the US). I love that the Slaver's weapon the kzin captain is holding is in "watermelon mode" in this pic. Of course, neither Arex nor M'Ress appeared in "The Slaver Weapon".
Tuttle & Bailey's first set added Arex and M'Ress to a scene with the aliens from "The Jihad":
The Jihad by
Therin of Andor, on Flickr