good question...
mine changes the further i get into the hobby and the more places/people/things i see...at them moment my fav's are probably either my N. jacquelineae, which i had coveted for as long as i've been into cps and finally got, my N. ventricosa, the first cp i ever got, my S. oreophila/leucophylla/alabamensis yearlings, which are either endangered or 5" tall, or my S. alata "white flower" that was picked out for me by Peter D'Amato at his nursery a while back. Thats not to say i dont like my others, it's just these are more special-er-er.