If you don't have any real preference and just want something that'll work well, then I would probably go with the Polks (I personally went with Alpines, but that was more due to availability than anything else, ntm I have the 2013 Shaker system so my fronts are 5.25" with a 1" tweeter instead of 5x7/6x8 like the base stereo). I know people also go with the Infinity Reference 6832cf, but I'm not 100% on whether there is anything that needs to be done for them because their mounting depth is a bit bigger than the Polks. I wouldn't say there is too terribly much different between the two in terms of audio quality using the factory head unit.
Replacing the front speakers is far more important than the rears since I am pretty sure the OEM head unit sends most of its power to the fronts. That's not to say it isn't worth replacing them, but if you want to save money and do far less work (removing the rears is a pain unless you have a convertible), just do the fronts.
Also, in case you still want to keep the OEM wiring, you'll want to get a harness. I got the Metra 72-5600 (note that they come two in a pack, I didn't realize this so I have some extra laying around). The only problem with them is that they seem to have reversed the wiring's polarity. It doesn't matter so long as you replace all the speakers with them, but if you only do the fronts you'll want to either cut and splice them to reverse the wires or make sure you buy them from Crutchfield because for some reason those aren't reversed. If you don't do this, the speakers will be out of phase.
2013 V6 Premium GHIG purchased 01/16/13