A bit under twelve years ago, Snoop Dogg complained about modern rappers sounding the same:
It's a bunch of dactyls.
If we observe XZARKHAN's STARSCREAM, we can see heavy usage of trochees, iambs, and single stressed syllables:
Most lines end with a stress, either naturally (i.e., the syllable is stressed with standard pronunciation) or forced (he emphasizes a normally-unstressed (or secondarily stressed) syllable in enunciation). I find this structure satisfying a a kind of chunky way. Like a brick.
It could be useful to study flow through this lens, since, while there aren't strict forms for rap songs like there are "traditional" poetry, it's likely that patterns emerge in both the preferred feet across a rapper's corpus, as well as how the structure and use changes with respect to the beat under the lyrics.