Snakes- I love handling non-venomous snakes. I'm not stupid about this option, I don't have a death wish, but the little constrictors are cool. They're much softer and silkier than most people (who haven't touched one) would think. Feeling them wrap around your appendages when they use you as a climb frame is actually quite exhilarating. And not in a life-or-death drama kinda way, but in a "woah, that thing is just one massive muscle" kinda way.
Pros - they are clean, once you've got the terrarium sorted; they don't need much maintenance and shed about once a month. They eat once a week, they poop (about) once a week. They don't need socialising.
Cons - they can live for up to 20 years, it's a massive commitment. Predominantly live curled up, in a box. Not particularly interactive. Most partners/housemates are not happy to share freezer space with pre-killed snake prey.
Chameleons - I just threw this option in 'cos I think these dudes are cool. They're just so... chillaxed, you know? They check you out sideways whether they dig you or not and they will not be rushed. Unless they're lining up to eat some tasty tidbit. But they're not big on being handled and when I did get to hold one in school, it chose to climb onto my head, using my earhole as a foot rest. I don't think I ever recovered from that. So I guess these are just on the list 'cos I wouldn't mind sharing a living space with one, nothing more than that.
Pigs - I've always loved the idea of having a pet pig. Just saying it makes me feel happy... Pet Pig. I firmly believe that when they're not living in a sty layered with their own faeces, pigs are probably very good pets.
Pros - They're highly intelligent, apparently easy to train (do you need more evidence than Babe? I think not). Tea Cup pigs are possibly the most adorable things I have EVER seen. Maternal hormones peak every time I see one. That's not weird, is it?
Cons - Apparently they are very big on pecking orders and will try to "subordinate" house guests by charging, barging and possibly biting. This could be awkward, especially if you want those guests to ever come back. Also, there's a small chance that you could unwittingly purchase what you think is a tea cup pig, which turns out to be a potbelly... Not cool.
Rats - I'd love a rat. I think they're fabulous. Again, similar to pigs, they probably make very rewarding pets when they're not scurrying through raw sewage. I don't understand the aversion to domestic rats. Why are hamsters so much more acceptable? They're less smart and less interactive - rats are more social in nature and bond easier with their owners, whereas hamsters are just... there. Running frantically in a wheel. Going nowhere.
Pros - it's not a long-term commitment, they live for about 3 years. While they might use you as a climbing frame (see Snakes and Chameleon), they're affectionate with owners. Apparently.
Cons - females go into heat about once a flipping week. No wonder you're 'never more than six feet away' from a city rat.
Cats - I'm not cat-person per se, but I like cats. I don't have anything against them. I love their haughty attitude and they make great pets, as long as you don't expect any gratitude in return. They amuse me when they try to pounce on reflected lights or lose shoelaces. As someone who can blow and catch bubbles for hours at a time, I can relate to this kind of frivolous fixation.
Pros - they clean themselves. They only poop in the litter tray (supposedly). They are relatively low-maintenance and can be left alone all day or even overnight.
Cons - the associated reputation. Just typing "cats as pets" into Google quickly brings up, "do cats have souls" followed closely by "80 ways to share cat pictures". Getting a cat might result in other feline owners thinking we have something in common. I can't be having with this.
Dogs - what's not to like? Dogs are social, affectionate, intelligent and inquisitive. They greet their owners, and generally guests, with enthusiasm. They almost always seem happy, or at least pleasantly surprised, to see you. Dogs respond to the mood of their owners and can be trained to play or calm down on cue.
It's hard to draw up a quick list of pros and cons. Depending on the type of dog you want, or the space/facilities you can offer, a dog can be high maintenance or can just fit in with its owner's schedule. Probably the biggest con for me, would be associated with people who think they are dog owners, when they actually treat them as animated accessories.
If you want a pet that size, get a guinea pig. Stop trying to pretend it's descended from domesticated wolves.