Mrs. Plant, a nice Jewish girl who views it as a kosher snack, Hellmann’s.
Me, a poor Southern boy whose family ate it because it was cheap and would do anything to disguise the taste, anything from pickles and onions to apples and pecans.
Mayonnaise, and nothing else.
Specifically, Best Foods (as Hellmann’s is known out West).
Mayo, minced onion, salt & pepper. That’s the way we like it at our house.
But some people really mean it when they call it “tuna salad”. I’ve seen it with chopped carrots, celery, pickles, flavoured with pickle juice (yuck) egg, celery, zuchini, apple, etc.
It’s not bad with the egg but the other stuff, not so much.
Mayo, celery and/or celery seed, crushed pepper. If I want to disturb my husband, garlic.
Mayo, celery, chopped green olives and capers. With lettuce and a slice of tomato.
Tuna salad is canned tuna, mayo, and sweet pickle relish of course, the only right way to make it.
Sometimes you degenerates, with your blasphemous and unholy ways, amaze me. 
Tuna Salad but we use salad dressing aka miracle whip.
Just a squirt of lemon juice and a pice of brodschen (SP? – my Deutsche is rusted solid!) on the side, eaten with wooden chopsticks.
Buster (the dog) likes his packed in oil, with no additional flavoring.
–SSgtBaloo
Pice? PICE?!?
Sigh
That’s what I get for concentrating so hard on the furrin’ words. 
Tuna
FINELY chopped onions and celery. I hate big chunks of celery.
Sweet relish if available. I can’t get it in Japan, so I finely chop some cornichon.
A zot of fennel powder. Just a zot, mind. This adds a wonderful savory note.
Kewpie brand mayonnaise. I am a fanatical convert to Kewpie brand mayonnaise.
Hollow out a section of baguette. Toast it lightly, so that the outside is lightly crisp and the inside merely lukewarm. Stuff with tuna mixture. Add tomato and lettuce if you are weird.
Eat with ice cold Coke and ruffled potato chips.
If any of the above ingredients are missing (including Coke and chips), it is a vastly inferior tuna fish experience.
Mayo, spicy mustard, sweet relish, Sriracha hot sauce, minced onion if I have any.
And drain as much water as possible, or it won’t be good.
Must be served on toast, or at least something more stable than regular bread! Lettuce and tomato are nice to have, but not essential.
First, the tuna must be albacore (NOT chunk light) packed in vegetable oil (NOT olive, NOT water.)
Second, only full- fat Hellmans mayonaise. Not lowfat. Not some other, pretender mayonaise. And not great big gobs of it, either; just enough to properly moisten the tuna.
Third, nothing else. Not onions, scallions, spices, celery. Nothing.
There are no specific requirements of the bread, but not all kinds are acceptable; see me for further guidelines.
Drained water-packed tuna, with mayo and chopped onion and green olives and garlic. Best eaten with sharp cheddar cheese on saltines or toasted homemade bread.
StG
Hellman’s. Although the suggestions of sweet relish are somewhat appealing. Anything like celery or onions is right out due to me never having fresh vegetables around. So a bottle of relish would work.
Copious amounts of mayo, either Hellman’s or Kraft. I use about a cup of mayo for every 3 cans of tuna, maybe a little more.
Chunk light tuna only, and only in water. And I drain the daylights out of it first.
MMmmmiracle whip and finely chopped celery and red bell pepper. Sometimes dill pickle if I’m in touch with my midwest roots.
I like my tuna with real, full-fat mayo, dill relish, chopped green onion, salt and plenty of fresh-cracked black pepper. I had to make the change from sweet relish to dill due to being on a low-carb diet. It’s not my favorite, but I still like it fine. And I serve a big scoop of it on top of a green salad and sprinkled with toasted almonds.
Mayo, and nothing else. All else is an abomination. An abomination I say!
This includes my dad’s love of putting a pinch of sugar in it.
A dap of mayo, black pepper, and some very crisp, finely crushed bacon.
Nothing.
(Well, ok, sometimes I sprinkle a little Lawry’s Salt on it.)
No frickin’ mayo, that’s for sure. Ew.
(Although I agree with GilaB that it must be albacore.)