This is an excellent question, but I find it hard to give a single answer. Traditionally, Opera has been the most standards compliant. For a long time, it was the ONLY browser to pass the ACID2 test in fact. FireFox and IE haven't been able to claim that (although supposedly IE8 is supposed to fix that, and FF is working on it all the time).
That having been said however, bear mind that IE has the largest "market share" of all the browsers right now (businesses have ties to MS, and Windows always comes with IE out of the box) followed closely by FireFox. So if your goal is to show off your app in a browser that most people will be using, it'll have to be one of those.
Purists will tell you that FF is more standards compliant than IE7 (and they are right), so that you should design for that and not IE. I can tell from many years as a designer/developer that pages taking that approach may not be a great idea. Bear in mind again - IE has the market share, and usually where it counts. So if it looks great in FF but breaks in IE, most users will be very upset, and the same vice-versa.
Best compromise - concentrate on those two. Tweak it to look right in at least FF AND IE, and now you've covered 90%+ of the people that will be using your website.
Don't get me wrong here - I'm not trying to dismiss the users of Opera, Safari, or any other browser. But if you want the most results for the least amount of work, then there ya go.
Best answer - take your time, do it right, test ALL the major browsers. The time spent working through these browser headaches ahead of time (when you can do it at your own pace) will be well rewarded. Compare that to the screaming client who wants to know why your page breaks in his favorite browser, and wants it fixed today. :)