well it all comes down to how much you are going to spend. less than $500, dont bother. $500-$700, AVIC X, or kenwood 6000 series. Over a grand =, then you should be looking at either a AVIC Z, a execlon, or a alpine.
for best sound quality, i would recommend a combination of an Alpine with the micro add on 4CH, but if you wanted this then you wouldn't think that the stock speakers would be fine.
also make sure you budget a good amount of money for all the extras.
dash kit and wire harness will be between $30-$40, the XM tuner will run about $100-$110, steering wheel controls adapter will be about $60, and if you need to have it installed, you probably be looking around $175-$250 to have it properly installed. hell if your going, you might as well go big and throw in another $50-$150 for a back up camera since pretty much all aftermarket radios will support it. so you could be looking at over $500 just for all the things to go with it.
i do recommend the AVIC's for best bang for the buck as their radios pack the most features for one of the lower prices (new X920's can be found online for $500-$600)