• The neck is a classic for a reason! Left side preferred purely because it's on the side the heart beats but it doesn't really matter which.
• The wrist is also pretty fun because you can just disguise that as a little kiss on the wrist
• Running down from the neck along the shoulders is fun for other sensual reasons, just not as easy to draw blood from there instead of, y'know, the arteries in the neck.

Ears get an honourable mention not for sipping reasons but just because ears are fun to nibble and bite on and watch the nibbled creature squirm

All of them!!!

Though a more serious answer is that I've not refined my palette enough I guess. I've only been sippin' for two years (exact as of today!). Clearly I need more, donations, to help me decide!

(AB- is nice for it's rarity but I really wanna try Rh null blood but good luck finding THAT)