A good ear for beats is pretty essential in my book; great beats can elevate a mediocre rapper to much higher levels. Conversely, rap music with wack beats is pretty unlistenable.

A discernable character or identity that shines through the lyrics and delivery is essential for me. I suppose this is individualality/originality but I can't stand lyrical rappers like Talib Kweli that have a lot to say but nothing to hold your attention. I would prefer a rapper like Scarface that just has so much presence on the mic that he can make technically superior rappers sound weak next to him. This makes rappers instantly recognisable and interesting in what can be an incredibly tired genre; other examples of individual/original rappers imo are E-40, Dru Down, Esham, ICP (and all of Psy actually), Ghostface, ODB, Bun B & Pimp C, 8ball & MJG, Ampichino, Rich the Factor, Tech N9ne....you get the picture

one word:

originality

or another word:

actual skills on the mic

Along with the obvious I'd have to say is great delivery, a lot of artists forget about delivery. It's called word painting, when you're voice get's angry with the lyrics or say something in a certain way to convey a certain emotion. Tech N9ne has definitely mastered this and that's why I think he's so great.

I like dark shit, occult shit, cult shit, and shit I can relate to. I also like shit that speaks truths that need to be told along with shit with hidden deeper messages.

I also like to hear artists that are diverse in the instrumentals and style they rap with. Put a little punk rock here, some blues and jazz shit there, some opera shit here and some gothic industrial shit there.