If you could only use one type of wood for smoking

That's a hard one :unsure:
I really have given this one a lot of thought, and think that if I only had ONE smoke-wood, I would go with apple. It will work with fish, fowl, red & white meats, cheeses, veggies....what ever!
OTOH, I love verity and have tried as much as I can...From aspen to grape vines.

i like hickory for barbecue, pork especially. believe it or not, against all warnings, i actually use the bark. it's from a shagbark hickory, and the stuff splits and falls off naturally. i get a ton in the spring each year. i have nothing against using wood, just haven't needed to both chopping any yet.

i like a hit of oak for roasted things, even a mushroom lasagna that we make. i'll often mix oak with hickory for barbecue too.

if i spot a couple twigs from our oak laying on the patio, i toss them in during a steak sear. there is no bitterness from bark.

i have a box of guava that i ordered off a guy here on the forum. so well packed, that two years later i still have some left. wonderful smell.

cedar shingles get sacrificed for planked salmon. i soak the plank to delay smoke until the last half of the cook, because the idea is not to smoke the fish, as it is to just have a hint of smoke. cedar has resins and tars, and you don't want a lot of that. normally you'd never use a softwood like that, but it's a small amount of smoke if done correctly, and in a way, that slight resiny note in the smoke works with the fat and oil of salmon, which is a 'strong' fish, fish-i-ness wise

i've used coconut shells also. my rule is "if it smells good, it will (likely) taste good.