Black Knight 2000 is my complete, 100% unbiased answer to this question .
Actually, depending on your tastes, I'd have to say its a tossup between Whirlwind, Pinbot, Taxi, and High Speed as the best *overall* System 11 pin.
BK2K is the perfect pin to me ... I love the ramps, loops, soundtrack, light show, taunts, and artwork. It takes me right back to the 80s every time I play it. It's not for everyone though. If you're going to have a single pin and have no emotional attachment to a particular title, any of the other 4 I mentioned are a blast to play over and over. If you happen to get tired of it, you can easily find someone to buy it or trade for something else.