Not much choice, but Uhura. Didn't care for the other two and Uhura wasn't given much to shine, operating the Enterprise's switchboard and rarely going on away missions or seeing action (that didn't involve being hit with the stupefaction ray from Nomad), but she conveyed quiet strength. She didn't have to be outspoken. She was tough but didn't carry the attitude with her most of the time (as Kira did).
In all of TOS... Helen Noel. Little contest. To make a list of top 10 of TOS I'd really have to think about it and watch all the episodes again. I'm only partway through a grand Star Trek rewatch on shuffle (I'd highly recommend it, even within just one series, if you've already seen the series in order. Let's you see differences in acting between the seasons, really noticeable in TNG, and judge episodes without the surrounding episodes weighing in on it (certain Voyager episodes are most changed by this, like "Flesh and Blood" without seeing "Repression", "Inside Man", "Body and Soul"). Just don't watch all the good episodes and then get progressively worse. Bounce around at random).