That is a difficult question, for even the most educated of Nook Color users, as it is very subjective.
I run CM7 nightlies (currently on #94 with Dal's oc kernal) and aside from some minor WIFI issues and the occasional SOD I think it's the best thing going. The latest nightlies are based on the 2.6.32 kernal.
Battery life, as I have mentioned in other posts is just insane, video is buttery smooth, bluetooth, the tablet tweaks are one of the best UI implementations I've seen for a custom ROM, the new "slide status bar" for brightness comes in at a close second.
I've had some issues with Netflix but that's really not much of an issue for me. That's what the 32in LCD is for, right?
All around I just love CyanogenMod. I have had it on my Aria for almost a year and it just keeps getting better and better.
Just my $.02, like I said very subjective. I think J515OP has a post listing all the ROM's current up to about a month ago if I can find it I will post the link.
I currently Have UltimateDroid 3.3.0 as stated in my sig, and I'm also running ADW Ex, as it seems to be the quickest/most responsive Home Replacement.