If I had to choose one single wood, Alder would be my all around, it is subtle enough for fish, yet works well on game, pork, poultry, and beef.
I personally prefer to use cherry, apple, citrus, and peach depending on the meat. These are my go to choices for smoking.
Someone on here once said, "Any wood is good for smoking as long as it's Cherry"
Humphrey's DownEast Beast W/BBQ Guru