By the power of truth, I, while living, have conquered the universe.
Hmm, it's quite a difficult choice because I actually like a fair few of them.
Kurt was developed beautifully, his character was simply brilliant. Kelly and Linda are the other S-IIs I liked, total badasses with a degree of interesting characterisation - especially Kelly, Nylund seemed to be hinting in GoO that she liked Kurt a bit more than what might be considered "professional".
For S-IIIs, it's Tom and Lucy all the way. They didn't get enough attention in GoO, but whenever they were the central focus you could always count on them doing something badass - be it blowing up a Covenant refinery or nicking the SPI armour from the DIs and gunning all of them down during a training exercise.