The "best" first time pet is most likely whatever pet appeals to your family for whatever reason
I agree with most of Panzer's advice too - plus would add that if you have one person in your family you *know* is allergic to cats I'd be leary of getting a cat thinking at least that person doesn't live with you (if they don't)... because we've found in our family that people who have seasonal allergies etc can find they also have more allergy symptoms when living with cats.
Other animals I had as pets as a kid were turtles, snakes, salamanders, frogs, parakeets and hermit crabs. Turtles are really easy - they eat salad-type food and can't get out of their cages. Parakeets were fun but really messy and as a kid I hated cleaning their cage. You might check and see if there is a reptile-pet-rescue group in your area
FWIW one of my ds' favorite pets when he was your ds' age was a plecostomus (algae eater) - we made many attempts at keeping a fish tank and were generally unlucky with most of the fish, but the plecostomus was great and really kept the tank clean!
Have fun figuring out who your next family member will be
polarbear
