Tenderloin (sometimes called filet mignon) is the king of steaks. It's lean, tender, can add any wanted flavor to it and it will absorb it all, and you can cut it with a butter knife. This is expensive though so only cook this once in a while.
For a quick and flavorful classic, saute bacon or pancetta and add onions, garlic and red pepper flakes. Cook with canned plum tomatoes for a mere 10 minutes, stir in fresh parsley or basil and toss with cooked spaghetti. Pass the Parmesan and enjoy!