Cats. Easily cats. I've always found them fascinating creatures, and I always had a Kind of Plan where I'd have a sparse bachelor pad which I'd share with a couple of moggies. My obsessive hoarding capabilities put paid to the bit about the place being sparse, and getting married to someone who's allergic to animal fur killed off the rest of the dream. *g* (But I adore Sarah and wouldn't change her for the world.)
Why? There's just something I find really fascinating about cats - they have a sort of inquisitive quality, they seem pretty intelligent (except the two I shared student digs with for a year, who seemed pretty dumb and dribbled everywhere, although they purred very loudly which was pleasant). I also like their independence, although I also like it when they let me pick them up and hold them 'cos they're very cuddly.
Two pearls of wisdom: Firstly, if a dog sits in your lap, it's because he loves you unconditionally. If a cat sits in your lap, it's because your lap is the warmest lap in the room. (Or in the case of Sarah's experiences with her sister's Sphynx cats, it's because you'd really rather they left you alone. Y'see - they're wonderfully contrary little things.)
Secondly: several thousand years ago, humans treated cats like gods. Cats have never let us forget that this is the way they still like to be treated...