Honestly it could be assumed that you meant the 8.4 since it's the only one with ROMs to speak of. I'm using the CM11 since it's been around the longest and is the only official AOSP. I'm hoping some of the unofficial ports get going better. My ROM of choice would be SlimKat if it were available.
Sent from my Slim Nexus 5 with smoothness by Faux.