Cats- Very little maintaining besides medical bills and spending some time with them, much more calm then dogs if you like that
Dogs- Having to spend lots of time with them and medical shiz
Fish- Cleaning bowl weekly, costly for filters and stuff inside the bowl
Birds- Ask flux I really don't have much expertise for them
Rats- I had owned rats for about 4-6 months before having to return them as school and my job wasn't letting me spend enough time with them but I have to say, if you want to spend the effort and time with them they can be really cuddly and can actually be taught to do stuff. Cleaning them is only like twice a week and you'd be spending around $200 for all their essentials such as cage, dental care, bowls and their own cleaning supplies (they clean themselves if they have water available to do it). So if your not grossed out by them I would recommend doing some research about them because they can be pretty cool and be a good conversation starter.
EDIT- Rats also need a second to not get lonely like guinea pigs, and don't get opposite genders because you will end up with more than expected
2 Males is good as long as they are family
2 Females is almost perfect as they will likely not have any issues with eachother even if they aren't related