What are you smoking?
Stronger woods for heartier meats such as beef, pork, lamb, or chicken: Hickory, Pecan, Mesquite (although many like Apple for pork as well)
Milder woods for fish: Oak, Apple
Also fish can be "planked" That is it can be cooked by placing the fillet (preferably a large fillet such as Salmon) on a thin cedar plank then placink fillet, plank and all over the fire (such as on the BBQ grill) to cook.