We're not prepared to say this is the "best" stopwatch app in the Marketplace. There are dozens of those out there and we have yet to take the time to give 'em all a go (though perhaps our Mr. Ponder would like the challenge?).
Either way, Stopwatch by Johnny Westlake (he also makes 'Bing Image Search' and 'Imgur Uploader' in addition to other apps) is no frills: it has a stopwatch (with laps) and a countdown timer. The app is simple, Metro and to the point.
But, version 1.3 just hit and it's been Mango-ized, meaning it can now run in the background--this is actually very useful for a stopwatch or timer app if you think about it! Only downside is during a countdown, if it runs out of time while the app is "in the background", it won't go off till you "resume" it. In other words, the countdown works in the background, but the alarm doesn't. Leave it on under lockscreen though and you're golden. But for a stopwatch feature in the background, it works perfectly.
And that's all there's to it. It's free so why not grab it here if you need it? Thanks, TheMoose76, for the heads up!