Usually just the salt, pepper and garlic and the pan juices after it gets seared, sometimes made into a reduction with some stock, or wine, a few more aromatics, butter and mushrooms may also be involved. It is simple and really tasty.
Or bearnaise. It is not so simple, and really tasty.
In general I sauce steak lightly, let the meat do the talking.
My #1 sauce would be a cream, pepper and brandy sauce. It compliments a rare seared sauce very nicely.
#2 Sour cream and mushroom. Both have nice deep, rich nice flavours.
I match these with rib-eye, porterhouse, rump or T-bone. Fillet is just way out of the budget here. Plus it lacks flavour. Any other steak I tend to use for oriental stir fry. With all the associated aromatics, like garlic, ginger, onion, and other items. Finished off with Hoi-Sin sauce or a dash of sesame oil, maybe oyster sauce.
The tougher less expensive cuts go into a stew or curry.
I can understand something like au Poivre or Diane, or London Broil, or even a Teriyaki as proper preparations of the meat but, to me, a steak is flavorful enough that it can sit on its' own without a sauce.
1) I love Pan seared filet with Goat cheese broiled on steaks for last two minutes of cooking and topped with Balsamic reduction sauce. I love this because its very simple, its beyond restaurant quality, and have made it so many times that I know it is good. Its a meal suited well for a elegant dinner party and doesn't keep you in the kitchen for very long.
2) A good Au Poive sauce is an absolute favorite of mine because I love when the peppercorns burst in my mouth and the creaminess just melts over the meat without over powering the flavor of the meat. Its the perfect compliment to a nice cut of filet.
As an aside, my wife and I just got back from our honeymoon in New York, and while we were there, we had the chance to dine at Peter Luger in Brooklyn. The steak sauce served there is unlike anything I've had before. Imagine a mixture of barbecue sauce and cocktail sauce, that's what it is. Much sweeter and with the addition of horseradish. It was a good sauce, but had no business being served with steak, IYAM. However, the steak itself was pure heaven on a plate.
Another tasty option with any sort of grilled beef is a chimichurri sauce - basically olive oil, minced garlic and fresh green herb like parsley or cilantro, depending on your preferences in that area. Maybe some dried chili flakes or fresh jalapeno bits or ...
- Second; beurre Maître d'Hôtel... Not really a sauce, it's simply herb butter made from butter, softened at roomtemperature, chopped parcely, crushed and chopped clove of garlic, some lemonjuice and s&p. Roll in plasticfoil, put in fridge. Cut a slice off and put on your steak. Absolutely delicious, a must with BBQ, also on lamb and baked potato! Put some extra slices of butter on the table in a bowl of water with icecubes to prevent them from melting and sticking together.