My favourite flourish to perform would probably have to be Molecule 2, it just flows so nicely, and retains the decks order, which is essential in performances. It also has the ace production and single card production to go with it.
Of course, as Vinnie already mentioned I love my own flourishes, there's 3 I'm very fond of that I've done.
But, as for my favourite to watch.....hmmm.....Probably a perfectly executed Hot Shot Cut. It's one hand and just looks so amazing to watch.
-RA69