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It really depends on your taste and what you are smoking. Some woods have a heavy smoke which is good for beef but strong for poultry. You can use any different of woods in your smoker. A lot of people mix woods.
I searched "types of wood" and here is a good thread.
In order I like apple, cherry, oak, mesquite, pecan then hickory. Oak is used mostly to mix with the fruit woods. We like a lighter smoke flavor as we have gotten older and find hickory a bit to strong when used straight and/or for a long smoke so it is used sparingly. We get our pecan and mesquite from our son who cuts it in south Texas for us. Strange but the mesquite seems light flavored while the pecan strong which seems reversed from what I have read.
I have to agree on the hickory and it's what I prefer on my salmon too.... even though lots of folks say it's too strong for salmon. And most people that try my smoked salmon say it's better than any they have ever had.