I have to say that even as a schoolchild neither the TOS Gorn nor the Horta were truly frightening.

I was watching monster movies on after school television, so the costumes for the Gorn and Horta were obviously that. I loved the Horta anyway.

The Gorn just seemed so inferior as compared to the Creature of the Black Lagoon and other contemporary examples. Time has only made the rubber suit more silly. In my view, the creature in TNG Conspiracy is falling into the same category now - memeable but not really scary.

But the Salt Monster I can agree was very disturbing. The fact that it was only shown briefly and there was no option for playback likely magnified it’s impact.

In terms of my own favourite, Andorians in Enterprise for the animatronics.

I like the Jem Hadar from DS9

Maybe a boring answer but Armus from TNG's Skin of Evil creeps me out with his voice and the music. But i've watched only TNG

I always thought Enterprise really nailed Andorians. They weren't the most extreme in terms of makeup and prosthetics, but they were convincingly alien and unique. I think the most impressive part was that the design made it believable that there were different Andorians who looked different. I suppose we quite literal saw this with the Aenar, which helps.

Tribbles are an excellent lesson in doing more with less. What an iconic creature that's nothing more than a fistful of rabbit fur on a string.

The false Balok from The Corombite Maneuver deeply freaked me out as a child, but heart forever belongs to the Horta

Slightly off-topic, but speaking of practical effects, if you haven't seen the SyFy reality game called Face Off, you really should look it up.

It's a practical effects makeup competition. The host, McKenzie Westmore, is the daughter of famed Star Trek makeup effects designer Michael Westmore, who also is featured in the early seasons as a "mentor" offering critiques and advice halfway through each episode's competition.

It's fun, the producers actively discourage catty fighting stupidity, and you get frequently see the competing makeup artists helping one another when it's crunch time.

Peripherally related

Well fuck, i read the question lol

I thought the vidiians were cool

Timeless.

Voyager crashes on the icy planet.

There is a video out there talking about how they did it

Borg are pretty cool looking. As a bonus their catchphrases have slightly sexual overtones. I bet a trip to Risa would clear up their complexion and mood.

that horny dog.