The worst dark side thing I did in KotOR2 was probably working for Czerka... I'd have been incredibly happy if Malak had showed up and bombed their HQ on Citadel... I hate Czerka, I do... so that was the DS act that hurt most for me to commit.
Now the DS act that I enjoyed most - I plotted quite deliberately (thanks to Aimo's pic of DS Exile and Disciple - really nice pic) to turn Di DS as quickly and as thoroughly as possible. So when I met him (at about 50-75%DS) I proceeded to say all the right influence lines - I actually saw the point in the conversation when I got enough influence points to turn him into a Force-user, because his hair went white, his eyes yellow and his skin nasty-looking. Then I offered to teach him to feel the Force, and I got the DS dialogue options for training him, and it said "The Disciple has taking his first steps on the path of Dark Jedi Consular" - I was so proud of myself. :D then I leveled him up, with loads of DS Force powers, and actually left the room I found him in.