Vinegar. Adding vinegar to things can brighten them up or, depending on quantity, give something a sour flavor that is needed sometimes. I experiment with all kinds of vinegar. I even use the brine from my jar of spicy pickled vegetables in my cooking or to make a salad dressing. I also like to swap it with lemon juice to see what happens. For example, I have learned that I prefer lemon juice rather than vinegar in my alioli mayonnaise I make (probably not original, but I figured it out on my own). Going in the other direction, I prefer vinegar over lemon juice for making tartar sauce. I think my vinegar discovery started when I was a little kid. In my home town, famous for its fishing, lobster trapping, and clamming, you'll find malt vinegar on the table at any restaurant. We use it on french fries and fried fish as a condiment. When I was a small child, I dipped my french fries in ketchup, splashed them with malt vinegar, and squeezed lemon juice on them. So, I think french fries are pretty much the vehicle to my discovery of ingredients. In turn, I have a tendency of turning condiments into ingredients I think I will dip french fries in just about anything on hand, at least once or twice.

It's basically the same for me! I'd say anchovies/fish sauce in general. My first introduction was through Thai fish sauce, which I grew up with and learned how to use in Thai cooking. Then, I started cooking with anchovies in oil and Worcestershire sauce in what I viewed as their respective cuisines. At some point I understood the similar role these ingredients played in providing the umami element needed to complete a dish, and that made me feel more confident using them in a wider variety of foods and "across" cuisines as needed. Used in the appropriate amount, they won't overpower a dish or turn it fishy; a little bit provides so much flavor!

Better Than Bouillon.

Anchovies! If you're only using anchovies for pizza and putanesca you're missing out on so much! Pop one or two of them in your tomato sauce right around when you fry the garlic before adding the tomatoes. It won't make them taste fishy but it will give it that extra something. Smush them and put them in your glaze! In your dressing!

Also less a single ingredient and more a mindset - booze. You need to use more booze in your cooking. I don't just mean "use more wine", I also mean different types too. Sake in your teriyaki, beer in your beef stew, cider in your pie filling, brandy in your stroganoff, kirch in your (sweet) pie, use it! Istg so many recipes I see online omit the booze or call it optional and it hurts me so much.

Smoked Paprika! Gives anything a quick/easy smoky flavor, with just a touch of heat (not like cayenne pepper). It's works with just about anything.

Since the classics have been mentioned already (vinegar, MSG), I'll go with tofu. It's like a blank sheet of paper, ready for your creativity. There's no form of cooking or kind of food that tofu doesn't fit into. You could have crunchy tofu cubes, but you can go sweet with silken tofu. Or make a creamy sauce with way less fat. Or honestly just be lazy and crumble in raw tofu into whatever your cooking to add easy protein.

5 spice.

That and brining poultry. A salt/sugar solution will do amazing things

Chicken broth. I started buying the cheap bone in chicken at the grocery store. Toss the leftover bones/fat/tendons into a crock pot and toss in the vegetable bits you would have thrown away otherwise. Cook it on slow cook while you're asleep and strain it in the morning. Now you have liquid flavour to add to rice, sauces, soups, etc., and it's full of vitamins, too

I've been getting into a recipe book lately that regularly suggests using the zest of a lemon/lime as well as just the juice. I can't believe I used to throw it away! It adds a slight bitter counterbalance as well as some nice floral notes to whatever I'm preparing.

Balsamic condiment. I hadn't realized just how much I was missing out on with more diluted balsamic vinegars. It's a completely different experience, which is great because balsamic vinegar still has its place in my heart for things like salad dressings. I couldn't even comprehend the balsamic + vanilla ice cream people until I'd splurged (slightly) on a mid grade balsamic condiment.

Chicken. Perhaps it is less the ingredient in this case than the techniques associated.

It's not that I didn't each chicken, but learning to roast a whole chicken was the gateway to real day-to-day cooking for me. I'd roast a chicken on the weekend, make sandwiches or salad with leftovers, make broth with the carcass, make soup or pot pie or gravy with broth and other leftovers, etc. Before cooking was something that seemed like a special occasion but now I had something that sort of begot more food naturally.

If I had to pick a single ingredient, probably cumin or curry spice if I can be allowed to cheat. So many dishes to add that to.

Schezuan peppercorns. I've had the same packet for years because it takes so little to add a very unique, peppery numbness to any Asian dish. Literally grind up a pinch in a mortar and pestle with some msg and put it in any Asian cuisine to elevate all the other flavors dramatically

Not an ingredient, per se, but for me the game changer was the flavor compounds from the Maillard reaction. Good browning is what makes some soups and sauces and meats and vegetables taste better than others. Regular supermarket cauliflower or broccoli or zucchini or ground beef or roast chicken taste way better just from proper temperature and moisture control (boiling water tops out at 212°F/100°C so the presence of liquid water tends to keep temperatures too low for Maillard reaction to happen). It's why grilling tends to taste better than, like, boiling, for many dishes.

Ghost pepper flakes, finally I can make things properly spicy without using a ton of really expensive hot sauce.

Some mf gonna come here and say cum

I’m not yet that guy who puts Tabasco on everything, but the future path is clear. I almost want to get one of those little Tabasco holsters to carry it around with me

Fresh lemon juice / zest. Just wow.

Tofu! I never got a chance to try it growing up, but when I started reducing my meat I decided to give it a try. It is possibly the most versatile ingredient in my kitchen, and by far the easiest way to get protein.

You can whip up a tofu scramble, throw it in smoothies/shakes, cube it and toss it in soup, fry it and serve with sauce over veggies, bake it with an herb coating to toss with pasta, even bread it to make katsu sandwiches! It takes on the flavor of whatever you cook it with, so it isn't hard to make the dish taste the same as if it had meat, and it is much more forgiving.

The trick is to get firm or extra firm tofu instead of medium or soft. Some people like super smooth tofu, but I find more people prefer the more solid versions.

An honorable mention also goes to nutritional yeast. It is often that 'something extra' missing from dishes that have a cheesy taste to them, and is a great option to add depth to dishes if you are concerned about MSG for some reason. I like to toss some on my popcorn to make it taste like white cheddar, but you can use it in any savory dish. It is also an excellent source of B12, which a majority of people are deficient in.

Maybe slighly off-theme, but:

Cucumber and spring onions

They are great on lunch-sandwiches (e.g. salami, cheese, cucumber, a few spring onions) and multiply my satisfaction with just 20seconds of extra work.

It's surprising how 2-3 slices of a tasteless thing can increaae taste so much.

Kosher salt, and by extension salting by hand with a salt cellar instead of using a salt shaker. Salt is some real basic stuff, I’ll definitely admit. But switching from table salt and changing up my salt game was a small detail that really got me into cooking.

I grew up in a house that was entirely table salt and salt shakers, so I didn’t learn about kosher salt until I started to learn more about cooking on my own. Handling kosher salt by the pinch and the hand made it much easier for me to develop an intuitive sense of seasoning food. If anyone is wary about over salting or doesn’t trust their salt shaker not to turn their meal into a salt lick, I highly recommend giving kosher salt and salting things by hand a try.

Adam Ragusea does a better job than I can at the moment of describing kosher salt’s context and advantages. I’ll leave the elaboration to him, but I’d be happy to give my personal perspective on details if asked. Apparently kosher salt is primarily an American thing according to him? I didn’t know that until reviewing the video for my comment.

Garlic chilly powder. An Indian mate of mine introduced it to me recently, and I reckon anyone who loves spicy food should have this in their pantry. You could use it as a spice in your stir-fries and curries, or sprinkle it on your fries/roasted veggies/pizza/pasta, add it to your favorite sauce or salsa, or just add some to whatever dish your making to give it a spicy garlicy boost. You can find this ingredient in Indian stores, if not, it's simple enough to make it at home.

For pasta dishes: Sun-dried Tomatoes

For tacos: pickled onions!

Habanero powder.. adds smokey spicy flavor so good

MSG. When I discovered how versatile and effective it is, it became a part of my daily cooking to the point that I keep it in a little spice drawer right next to the salt and pepper.

Lately, honey fermented garlic. It was a giant leap forward for my stir fry and sauce game. Starting to branch out into more honey-based ferments.

Yellow onion. I basically always have at least one or two on-hand and incorporate them into basically all of my singature cooked dishes.

Ooooo, this is a good question!

I think mine would be wasabi (or horseradish, depending on how much you are spending). So delightful.