Oh, definitely bacon.
Club Sandwich:
White toast
Mayo
Lettuce
Thinly sliced turkey breast meat
Crisp bacon
Sliced ripe tomato
I don’t like the third slice of bread, so I leave it out. It takes up space that should be devoted to more bacon.
The first thing I thought when I saw the OP title was “…and I’m not even a member.”
Not just bacon but crispy bacon. Don’t be puttin any of that jello-lookin’ stuff on there. It’s gotta be brittle.
I’m sorry, those were Beggin’ Strips.
Well, that’s wrong right off the bat.
The original club sandwich was made with sliced chicken. Turkey was substituted to save bucks back during the '50s, when most of the Grand American Traditions went straight to hell.
Keen’s English Chop House still makes 'em the right way.
But not raw bacon. The manager thanked me when I pointed out that the prepared sandwiches in the deli case contained - not partially cooked or merely less than crispy - but raw bacon.
Clubs *may *have ham instead of bacon…
not to say that I would be ordering one like that… 
From The New Food Lover’s Companion, Sharon Tyler Herbst:
This is called a Turkey Club Sandwich.
The generic “club sandwich” is any sandwich with three pieces of bread and meat, usually cut into four pieces with silly toothpicks. Yes, a Turkey Club Sandwich must have bacon on it. No question. But there are many varieties of club sandwich which do not require bacon.
My wife was delighted when our favorite pub returned the turkey club sandwich to the menu, but irritated that it no longer included bacon. I pointed out that other sandwiches on the menu had bacon, and that they could surely add it.
The waitress grudgingly agreed that she could, but warned us she would charge us extra. Whatever.
Then the bill came: she also charged us extra for the cheese that came on the sandwich, no matter that the menu listed cheese as an ingredient of the sandwich.
I don’t normally argue with waitresses, but she pissed me off. The weirdest part was that, when I pointed out the mistake to her, she refused to even look at a menus, just insisted that she was right. No good tip for her!
Daniel
*Waitress: You want me to hold the cheese, huh?
Daniel: I want you to hold it between your knees. *
I almost always see bacon on club sandwiches, but I agree that the defining characteristic is three slices of bread. Everything else can vary.
A chef’s salad has to have some kind of meat, usually turkey or ham. Otherwise it’s a garden salad.
Wait, now you’re all quibbling over whether a club correctly has bacon in it. . . but THIS club sandwich had TUNA!? At that point already unless we’re going with the three-slice-of-bread definition. . . I mean, tuna. Come on. A “Tuna Club” and you still expected something specific?
What, you’ve had them without lettuce&tomato?
I’ve always thought of a club sandwich as a kind of BLT on steroids.
I must live in the same parallel universe as Trunk because I agree that a club sandwich, by definition, has bacon on it. Three slices of bread. Pffft! I order club sammiches all the time and none have three slices of bread. (Think of the carbs!) But ALL have bacon.
So, Trunk, did the waitress throw on some bacon or was she content to just argue with you about it all night?
Waiter! There’s no bacon in my club soda!
:dubious: Here it comes. Open your mouth and close your eyes.
I always have them without tomato. I hate tomato.
Cheese on a club sandwich? Now you’re just being silly! 