What is the prefered type of aquarium driftwood?

It doesn't matter what type of tree the wood comes from, as long as you boil whatever wood you get. Keep boiling the wood until the water it's boiling in stops changing color and boils clear (this will take several boils. You'll have to boil it in clean water again and again and again). If you keep boiling the wood until the water it's in no loner turns brown, then the boiled wood will not dye your aquarium water brown, not noticeably with normal weekly or biweekly water changes.

I recommend the wood from the beach that has been rubbed to an interesting shape in the ocean. That kind of wood, "driftwood" is nice-looking and takes less time to boil clear since it's been soaked in water already. If you use wood from your local forest, then it'll take you longer to boil it clear and it might not be in as interesting of a shape.

The only reason people go out and buy driftwood from petshops is because they don't want to take the time to boil the driftwood or because they live in a landlocked area and have no other access to driftwood. I would just boil my own wood and save the money.