Day One: Without question this is Discovery - The Vulcan Hello.
This was the episode that brought me back to Star Trek, the trailer alone blew my mind. Seeing Michael Burnham and Captain Georgiou working together - their trust and companionship, the desert landscape and desert community, the technique of walking to draw in the sand, the appearance of the ship from the skies.... Even without what comes next this was a spectacular opening episode that set up so much in terms of character and scope of vision. Just completely wonderful! 💫
Day Two: It says it in my bio and that has not changed. My Captain will always be Captain Michael Burnham. 🪐
Day Three: This is more difficult as I honestly think there's loads!
I think Joann Owosekun and Keyla Detma were very underrated on Discovery.
But to give a non-Disco answer, I'm going with B’elanna Torres from Voyager. To me, it's still totally unjustified that she was remotely interested in Tom Paris and I think a lot of screen/plot time was wasted there.