I love all of these, but if I could only ever live with one it would be capers.
Pickles a close second. My FIL has an amazing recipe for tuna salad: three ripe avocado, three tins of tuna drained and pressed for minimum moisture, three teaspoon mayo and six small pickles finely chopped.
(3 tsp mayo is about 1tbsp, but i wrote it like that so you can adjust to size needed 1:1:1:2)