I hot smoke year round and cold smoke in the spring and fall.
For fish (hot smoked) I prefer alder.
If I'm cold smoking fish I often mix alder and apple.
Cheese, cold smoke of course, hickory.
Salt, cold, hickory. Provides wonderful depth to foods. Love it on eggs and it provides a necessity...blood pressure.
For brined birds I like a mix of apple and alder. I did a taste test at a family gathering where I smoked two turkeys. Both were brined with the same mix. There were about 20 people and only one preferred the hickory smoked bird over the alder/apple smoked bird. Both were a hit an there wasn't any leftovers of either bird...
Also on birds, making broth from smoked chicken/turkey carcass is tops. In the freezer section there is a brand of noodle called Reames...folks will think they're home made.
For prime rib, alder. Outstanding with just salt and pepper on the meat prior to smoking. You can bring prime rib to room temp before smoking for a more uniform doneness or leave it in the fridge if want a more rare center. I try to keep prime rib in the 180 range and then crank it up for the last 30 minutes or so -easy to do on big green egg.
Not a true 'smoke' but for stuff cooked relatively hot/fast (burgers/steak/fish/chicken boobs) I'll add smoke to supply some depth of flavor, I'll use hickory. Hickory is strongish and works great for short cooked foods.
Leg of lamb (a favorite), mix alder and apple.
All of cooking is about personal preference, the experimenting to get something the way you want it is fabulous good fun and it didn't loose any fun after 'perfecting' a processes IMO. Enjoy the ride...