Step 1: Leiomyomas are benign uterine smooth-muscle growths, and because their blood supply often fails to keep pace with their size, they frequently undergo secondary degenerative changes.
Step 2: Among all such changes, hyaline degeneration is the most frequent. The cellular and fibrous components are converted into a uniform, glassy, pink-staining substance, and this process is the one most commonly encountered in routine practice.
Step 3: Ranking the alternatives, calcific (calcareous) change tends to occur late, especially in elderly or postmenopausal patients producing a hard womb-stone; red (carneous) degeneration is the pregnancy-associated form and is comparatively rare; cystic change generally arises when previously hyalinised tissue breaks down and liquefies, so it is downstream of hyaline degeneration rather than the leading type.
Step 4: The commonest degeneration occurring in fibroids is therefore the hyaline type.
\[\boxed{\text{Hyaline degeneration}}\]