Firefox, Safari, and Opera are all quite good (as is anything based on them; Flock, Omniweb, etc...). Each has its quirks, but in general they're relatively minor quirks (a few examples: firefox doesn't support display:inline-block;, opera 8 doesn't support the CSS DOM apis, safari has some issues that prevent most web rich text editors from working with it).
IE7 was a huge improvement in web standards support from IE6, so at the very least make sure you're using something based on it. I'm looking forward to seeing what happens on the web as it becomes common.