Don't know why I did this, but I searched the names of the 54 sidrot in the Torah for the presence of the letters in the Hebrew alphabet. Every letter appears at least once, but some are fairly rare. Here are the "winners" on the rare scale.
Gimel appears just once (ויגש)
Samekh appears only twice, and very close together (פנחס, מסעי)
Two letters appear just three times
zayin (תזריע, האזינו, וזאת הברכה) --- and 2 of them are the last 2 in the Torah
tet (משפטים, מטות, שפטים)
Honorable mention goes to the letter kaf, which appears four times, but three of them are in the word כי, which some people omit from the name of the sidra in informal conversation:
כי תשא, כי תצא, כי תבא, וזאת הברכה