I'd say the best-looking computers were the M-5 in "The Ultimate Computer" and the Beta 5 in "Assignment Earth,"
What? No Rayna Capek from "Requiem for Methuselah" or Andrea ("Androida"?) from "What Are Little Girls Made Of?" Alice from "I, Mudd"?
Having to portray things way beyond current technology prevented Star Trek from using much real hardware.
That is a constant concern for any sci-fi. I've read that McCoy got his "surgical salt shakers" after it was decided the props would not be recognized as
salt shakers for the episode "The Man Trap." Even today, many of the "computers" in TOS strike me as nothing more than controls and lights (following the "PC Props" article linked above). Those are mere control panels, not "computers."
And there are other considerations. Many times in TOS the audience sees textual data displayed on a computer screen that is actually the image of a typewritten page—with penned underlining, as seen in "Where No Man Has Gone Before." Computer fonts have improved as display technology has increased, but even then there is a limit to how much can be comfortably displayed on a video screen. While working as a video engineer in the '90s, I recall student journalists often using a shot of a local paper as filler for a story. Newspapers are an entirely different breed from video screens. And in order to get an entire headline in frame, the student reporter would have to zoom out so far that everything else turned into gray mush. I was constantly telling the reporters, "This is a very boring image. Try to think graphically. Isn't there anything
else you can use?"
Today's audiences are a little more sophisticated. Also, big cinema screens or HDTVs allow an actor to casually display fictional technology in action without requiring an overt close-up to insure the audience sees it. STAR TREK was trying to depict technology 300 years ahead in terms that would be recognizable to '60s audiences. The communicators from TOS were very small and streamlined compared to walkie-talkies from the period. TNG introduced the comm badges, which also looked slick then, but entirely practical and possible now. If TOS had leaped ahead of even Google Glass and posited "cyber brain" implants like those from the GHOST IN THE SHELL series (which today's audiences would recognize as akin to built-in "smartphones" with Web capabilities and HUD augmented reality), would '60s audiences have understood it, even with an actor stepping out of character to explain it with a long dissertation on the technology?