I'm talking about the aspects of their rapping, not their imagery, storytelling or message.
We often use a lot of ways to judge a rapper's technique, but hopefully we can clarify the difference because a lot of good rappers are good in their own way.
Lyricism - a lot of multis, complexity, word play (basically, the words)
Flow - the speed and division of syllables (how the written lyrics are to be pronounced)
Delivery - their voice, intonation, syncopation (the pronunciation of the flow)
Lyricism varies. And depends on flow and delivery. Plenty of underground rappers have great lyrics, full of multis and wordplay, but they just can't flow or deliver them. Then you have rappers that lyricism and those other qualities, and become successful like Eminem or Nas.
The flow for example would be Notorious B.I.G. or Kendrick Lamar. They did not have complex mind-boggling lyrics and if most rappers tried to perform those lyrics, they just wouldn't have thaw flow. But Biggie for example can take the most simplest lyrics and make them into a hit. He didn't need a large vocabulary or wordplay. Lamar could take a lot of syllables and balance them perfectly.
The delivery for example, would be Snoop Dogg. His lyrics are very simplistic. His flow was weak. The average person could take his lyrics word for word and pronounce them the same way he does, but without that voice, it won't come out too good. Snoop could take the worst rhyme possible and make a great song out of it with his cool and calm voice. 50 Cent and Royce da 59 both have great voices, but if an ordinary voice were to rap their rhymes (even with the same flow, it would suck).
The lyricism is the first step, the cradle. The delivery is the final touch, but perhaps the most important one of all. The flow is in the middle. The flow borrows from the lyrics and the delivery borrows from the flow, so that is the central aspect.