What is a good steak marinade or seasoning?

I need information here. How are you cooking the steak? What cut of steak? How does it grade?

The only acceptable seasoning for a thick, high quality cut of steak is WHOLE black pepper and kosher salt. Press the peppercorns into the meat. Let the flames hit the steak while it cooks to add a nice char, and let it rest for 20 minutes before eating. The juice that accumulates in the plate as it rests is the only sauce you need. When I'm cooking my own beef this is near exclusively what I use. If you have a 2 inch thick prime/high choice ribeye this is perfect. Works great on KC strips and sirloins too.

But if you're working with thinner cuts or lower quality meat, there are a few other seasonings I like to use.

For something like a select/low choice thinner steak (like what you'd get at walmart) I have a specific recipe I like. No specific measurements though, just do what your heart feels is best. First you mince some garlic and onions. Go easy on the onions. No, you can't use the dried shit. Then add brown sugar. Then paprika. Take your meat and add an appropriate amount of salt and cracked pepper. Then rub it down in your paste you've made. Throw it in the fridge overnight. Grill it like normal.

If you're doing a steak in cast iron, follow the other people's advice in here and go salt and pepper at first, then butter, herbs, and garlic to baste right before you're done. I'm not a fan of steaks in cast iron though, you miss out on a good char.

If you want to cook a flank or skirt steak for something like fajitas, mix a cup or so of orange juice, a few limes worth of lime juice, salt, pepper, cumin, chili powder, paprika, garlic powder, onion powder, and cayenne to taste. Slice the meat (this works for chicken too) into fajita sized pieces, put it in a bag, and pour this mixture over it. Let it sit overnight but no longer than one day. You can add sugar if you want but with the orange juice it really isn't necessary.