I find pretty much any sport at a high level is watchable, once you take the time to understand the rules. So I can (and have, and do) enjoyed NFL, NBA, NHL, MLB, Aussie Rullies, Rugby Union, Rugby League, pretty much any Olympic Sport, Tennis, Golf (Ryder Cup is great, The Open is the best major), F1. And anything else I've forgotten. Pretty much the only things that I wouldn't watch are boxing and horse racing (you have to be there to enjoy that)
But the number one for me is Football (the proper version, where you kick a BALL with your FOOT :) ). No other sport has such a dynamic flow, combination of team work and individual skill. A great team is always greater than the sum of it's parts. Every goal is vital. It takes seconds to create a move the length of the field. The tactics and strategies are fluid and evolving.
And it has the highest chance of an underdog win of any pro sport (I'd find the link for the research for this if I could).
And, finally, it's the simplest game out there. All you need is some ground, and a 'ball', and some people. That's it. It's why it's the most popular team sport in the world. It can be played in a field, in the street, in a garden, in a park. You can play with 2 a side up to dozens. It's not increasingly limted to those at the extremes of the human body shape range (hello NFL & NBA).
Yes, it can be dull, but so can any sport. Some of the best games I've seen finished 0-0. And that's fine. But I'll never forget Maradona's 2nd goal vs England in 86, Zidanes volley at Hampden, Bergkamp at Euro 88, Owen vs Argentina, or many other brilliant moments.
The greats of the game are known worldwide - Pele, Maradona, Puskas, Cruyff, Beckenbauer, Zidane, Messi, Ronaldo.
No game captures hope, despair, elation, defeat, joy, pain, glorious victory or shock as well as Football. Nothing ever will.