One might think that Darwin's favourite animals were the so-called ‘Darwin's finches’. In my view, Darwin's favourites were without doubt the cirripedes (barnacles). Indeed, Charles Darwin spent as many as 8 years (1846 to 1854) studying barnacles, eventually writing two monographs on extant cirripedes, and two shorter ones on fossil cirripedes
[1],
[2],
[3],
[4]. He was so involved in this work that, concerning a gentleman neighbour, one of his sons asked:
“where does he do his barnacles?”, as if studying barnacles were every father's main occupation.