I think it depends on the beef really and there is no definitive answer. I've had steak cooked medium well by accident and it was great. I've also had steaks cooked rare and it was bland and uninteresting. Generally, I go for rare or medium rare. My favorite rare steak was the one I've had in Paris. It's very popular to have steak rare in Paris but I think it's because the beef is different and works well with that cooler temperature.
Also, in some cultures, eating steak well done or medium well is common. For example, in the Latin American cultures, they eat their beef fully cooked or at least past rare. I like that too. Basically, I love meat so much that I like it in any form. I could bite into a cow and still be happy.