What is the simplest possible marinade recipe?

The "Simplest" is probably just a salt rub. It works on almost any meat, and will elevate the taste. That's not a great answer though, so I'll explain what I think of when I'm making a marinade, and the order in which I think of them:

• Oil- just about any oil can work as a marinade base. I like Olive Oil.
• Salt - this is an important part of cooking in general; salt makes things have more taste, and it's important to work it in early.
• Sweet - I like maple, honey, or brown sugar. They're all easy to add in and get something out of.
• Acid - apple cider vinegar or lemon / lime are both good options.
• Aromatics - herbs, garlic, onions, etc.

If you start with this as a general outline, you can make any kind of marinade that you want. You can also make some shortcuts, like using ketchup as both sweet and acid, if that's something that you like.

Not a meat eater. Vegan. That said:

There's brining and there's marinating. Marinating usually involves an acid, IIRC, whereas brining indicates the presence of salt so as to migrate salt across cell walls to penetrate into the food. We tend to use the word for anytime we soak an ingredient.

Pedantry aside, I like:

• Soy + a citrus juice
• Soy + rice vinegar or Chinese black vinegar (I like this a LOT with thinly sliced cold form tofu with sesame oil, black pepper, granulated garlic, and chopped green onions as a meal—think tofu dumplings that's just tofu)

Are they marinades? Brines? Yes? I'll go with yes.

Balsamic vinegar can be good for some marinades. Mushrooms work well with them.

I think in general, lemon juice, olive oil and soy sauce as a base. Modify the ratios depending on the profile you want, and add herbs and spices.

I like to cook a lot of asian dishes, so a lot of my marinades are soy sauce, brown sugar, garlic, rice vinegar and usually a bit of sesame oil.

I think in general you'll want to modify between different types of meat. Here's an easy one that you can take in a lot of directions:

1/2 c olive oil
1-2 garlic cloves
half a lemon worth of lemon juice
salt & pepper
oregano and/or thyme and/or basil (fresh is best)

Works great for poultry as is. For beef, add a few tablespoons of worcestershire and a 1/4 c of soy sauce.

I'm curious why you are set on a marinade but leave the rest open? Aren't you just looking for a tasty, simple recipe?

Marinades are essentially super easy, it's an oil(or more precise a fat) with flavoring and salt, and maybe some acid.

Generally when I taste marinades they should taste strong, a bit too much maybe because they are meant to impart flavor, they're not a sauce. Some chicken marinades use Greek yogurt or buttermilk and those are a bit different.

Some examples:
-olive oil, black pepper, garlic, green Italian herbs: general Mediterranean for most cuts basically. Maybe add some red wine vinegar if tougher cuts of meat.
-greek yoghurt, lemon, olive oil, lots of garlic, oregano, red wine vinegar, pepper, paprika and green herbs like parsley or mint: love this on chicken
-soy, neutral oil, black pepper, garlic, lemon: very basic Asian style.

My point is I don't really have recipes for them I just add flavors as I feel like and what I have, keep tasting it while adding things it's fun and it grows your cooking instincts. It's fun making sauces and marinades.

Water, some kind of vinegar (or an acid like lemon juice), olive oil, salt, and pepper. Seasonings and aromatics are up to you.

The ratio of oil to acid is around 3:1. Marinate chicken and pork from 3 to 12 hours. Beef from 3 to 24 hours. If you leave the meat in too long it gets mushy.

A solid marinade to start with is:

1 cup olive  oil

½ cup soy sauce

⅓ cup red wine vinegar

¼ cup fresh lemon juice

3 tablespoons Worcestershire sauce

1 tablespoon freshly ground black pepper

2 tablespoons Dijon-style prepared mustard

1 onion, sliced

2 cloves garlic, minced

From: https://www.allrecipes.com/recipe/18011/marinade-for-steak-i/

Plain yogurt makes an amazing marinade base that keeps meat moist. Tandoori is a popular take on this, but there's so many other variations you can make. A personal favorite is taking plain Greek yogurt and mixing it with apricot preserves. The sweetness of the preserves helps balance the acidity of the yogurt and the yogurt keeps everything juicy. Here's a more in depth recipe that I've been using often from Olia Hercules:

• 1 Tablespoon neutral oil
• 3-4 Tablespoons of chopped herbs or dried (Dill works really well), add more or less to taste
• 1 Cup plain yogurt, full fat preferably
• 3-4 Tablespoons Apricot Preserves, though Olia's recipe originally calls for Honey
• 2 Teaspoons hot sauce or omit
• Salt and Pepper to taste

You can leave out all the extra stuff like the herbs and hot sauce, but the base of oil/yogurt/fruit preserves really offers a lot of flavor. It works best on the grill, but always comes out perfect in the oven as well. But if I have chicken, it is nearly impossible for me to use any other marinade because this one always delivers something so delicious!

I have one that works on chicken, pork, and beef as a fajita-style marinade. The original is from Serious Eats and I probably make it 3-4 times a month.

1/2c low sodium soy sauce (regular is too salty for this but if it’s all you have, drop it to 1/3c)
1/2c lime juice
1/4c brown sugar
1 tsp pepper (fresh ground ideally)
2 tsp cumin
1 tsp chile powder
1-2 cloves garlic, minced or grated
1/2c canola oil (or other high temp suitable oil)

Mix everything but the oil, add the oil, mix thoroughly again and pour over your protein of choice.

Marinate in a sealable plastic bag or covered container in the fridge for between 1 and 24 hours, give it a shake occasionally, grill or broil to taste, and that’s essentially it.

If you are broiling it instead of grilling, I suggest cutting back on the oil to like 1/4c or so because I have found it comes out kind of oily otherwise.

Everyone else's advice is good. Unlike some others, I like to make the marinade in as close as possible to order of liquidity, though - starting always with the oil (olive oil), which is the base and the most abundant ingredient, then likely I'm going to add the acidic ingredient, since it's always a thin liquid (any citrus or vinegar that feels right, there are many variants of both). Then the salt, which can be soy sauce instead (liquid). If I add anything sweet it will likely come at the very end; instead at this point I'm going to consider whether to add mustard. Mustard is great, and no one is talking about it! Some kind of hot sauce as well, if you want spicy. Then the remaining herbs and spices you choose to include (solids), and finally the sweetness - some kind of sugar, or maybe honey. It's easier to mix the honey in if there's already a lot of liquid in there.

I recommend having at home:

• Olive oil (worth getting the good stuff)
• Some kind of white vinegar
• Some kind of balsamic vinegar
• [EDIT] Some kind of crappy wine or beer (never found much of a reason to overspend on marinade wine)
• Soy sauce
• Mustard
• Coarse salt
• At least one type of hot sauce
• Honey
• White and brown sugar
• Peppercorns
• Purchase fresh citrus if you're going to use it, but it's worth buying them regularly in any case; you can always make a juice before they go bad.
• All the dried herbs and spices you can get your hands on; they last for a very long time. Note that some fresh herbs differ considerably from the dried versions; you have to buy those fresh if you're going for the fresh flavor (chives, parsley, basil, ginger, etc.) I usually have dried: oregano, thyme, basil, cilantro, parsley, tarragon, rosemary (delicious!), cumin, turmeric, paprika (important!), cinnamon, nutmeg, ginger and cloves.

I suppose the simplest marinade uses any two ingredients. Olive oil and soy sauce? Every combination you make imparts its own flavor to the meal. I basically make up a combination on the spot each time.

Remember you can add extra flavor with a bay leaf. Bay leaves are great and also have a very long shelf life. Take it out before eating though.

If there's leftover marinade mixed with meat juices you can turn that into a sauce. I like to heat it up with butter, mix well and let reduce then add cream or some other base (soy cream has a very long shelf life).

I like that a lot of people are talking about using Lemon Juice and that my go to as well but when I travel I usually just pack orange juice because me kid likes that. It's simple enough and still works great.