Sadly, the most important thing is the beat he's rapping over. I can give a couple of examples.
When Kool Moe Dee and LL Cool J were going at it, I thought LL had better beats. I bought Kool Moe Dee's Go See The Doctor album and I wasn't feeling his production at all. In contrast, LL's Radio and Bigger and Deffer albums had stellar production so I preferred LL. I met Grandmaster Caz at a Zulu Nation Anniversary party circa 2005/6. We were discussing the LL/Moe Dee battle and Moe Dee was his generation while LL was my generation. He favored Moe Dee and said he thought Moe Dee won the battle. I told him that I thought LL won because he had better production.
Bottom line; the music has to go through your ear to get to your brain before you can analyze the lyrics. If the beat isn't banging, I'm not listening.
Another example is Keith Murray's second album. I was in my homeboy's car listening to the album and the beats were dope, but he was using made up words and wasn't really saying anything. It was really unlistenable, although the beats were dope.
As far as actual MC skills, I like cadence and wordplay. One of my favorite verses is Big's verse on The Benjamins. He said, "Fuck the state pen I fuck hoes at Penn state". It's not just the wordplay, but his cadence.
But out of all the choices listed, it's really a combination, and the beats.