Cats can be be very annoying and stuck-up, they seem to believe that the world revolves around them. Plus, go up to anyone with a cat and you will see numerous claw marks on their arm.
Birds don't really do anything except make lots of noise, they drive me crazy. If you are one of those people who deeply dislikes going into pet stores, birds are probably one of the main reasons.
Fish are much more calm, but they are to calm. They just swim around all day. The only interesting thing that happens with fish is when they poo and it trails behind them for a while. But even that gets boring.
Reptiles probably come in second for me. They are almost as boring as fish, but not quite. They also look pretty cool.
That leaves..... dogs! If you don't mind picking up poo and you train them good, they are a great companion to have. They can easily brighten up anyone's day. I vote for dogs.