Hmm... the favorite that I've got is probably N. lowii, spec. x vietchii, chaniana x vietchii, or eymae. None of them are doing anything really interesting or impressive at the moment, though. My "favorite" in-collection probably has to go to my N. sanguinea and alata x ( alata x ventricosa ), which are both showing adult traits for the first season ever. My N. ventricosa x inermis is also looking pretty svelte as of late.
My all-time favorite is probably N. sibuyanensis or peltata. If N. aristolochioides were a tiny bit easier, it'd probably be that or a hybrid with it.