Well, the sentimental favorite is the salt monster, because that was the first Star Trek episode I ever saw.
The Horta holds a special place because: 1) it brought the possibility of silicon-based life to the mainstream; 2) it taught a whole different level of understanding of "monster" (it was just protecting its chilidren!); and 3) they used the Mangani term in the Tarzan books for "boar" - not that anyone noticed.
But my real favorite Star Trek monster would be the Borg. I mean, talk about an unplacable, seemingly invincible enemy. That creation took "adversary" to a whole different level. What if you're facing destruction ("assimilation") by something like that? Great creativity there.
Here is the Borg for those not familiar..
looks like a real rip off of the cameraman in Hellraiser that was turned into a demon by Pinhead..
I never thought about the Horta name like that before,even though I was an avid reader of the ERB Tarzan books as a kid! ha,that is funny,don't know how I missed it..after all,how many times did Tarzan make a meal of Horta,then wiping his greasy fingers,curled up in the bole of a great tree for a nap..
Another implacable enemy was the Doomsday Machine..though not very visually stunning,it looked kind of like an old hollow dog turd cruising through space,eating everything up..