First of all, don't waste your time with lettuce. Use cucumbers for a BCT.
The way you make a BCT is with bacon, home-grown tomatoes (otherwise stop and just eat the bacon), mayo, cucumbers, kosher salt, and toasted bread.
To answer the question directly, the two most important ingredients are bacon and home-grown (equivalent) tomatoes. Another important ingredient is added kosher salt sprinkled on the tomatoes to bring out the flavor.
Kosher salt? Yes, so you don't get that metallic iodine flavor on the tomatoes that you sprinkle it on. B-b-but, shouldn't the bacon and mayo already provide enough salt? Not with the cucumbers and tomatoes. You need that extra bit to bring out the flavor of the tomatoes. Just a sprinkling. The cucumbers will ensure that it isn't too salty.
Home-grown tomatoes (or flavor-equivalent if you can find them) are a must. Otherwise you're just adding flavorless mush to your sandwich, and you should just omit it altogether and just have a bacon sandwich. Tomatoes have natural amounts of glutamate in it, which also enhances the flavor in conjunction with the bacon.
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