What is the best software (easiest) to use with the Arduino Mega Pololu Shield (RAMPS) kit?

To get up and running quickly, I recommend RepSnapper. The Linux and OS X ports are probably more stable right now, but the improvements are making it slowly into the windows version. There was a Beta version for windows released recently (last four weeks?) that is not really useable (at least that's my understanding). I use the windows version just prior to that release, and I have seen another more recent release that I cannot locate now (someone please comment if you know where this is).

Ben Jackson has a version of ReplicatorG that he's been modifying to work with FiveD style firmware. You can download his work at [ben.com]

For firmware that works well with RepSnapper, I recommend caruKlip / klimentkip. These are merging, and possibly identical at this point. It supports acceleration in firmware (you should have acceleration turned off in RepSnapper, as this is software based acceleration).