Well, I've got all three you're asking about. The G6 is by a wide margin the best of the lot, if image quality is your primary concern. It's also the nicest to handle, and has the best control placement. The G5 does most of what the G6 does, though it's a little clunkier to use and handle, and I haven't had any real difficulties with the CA issue raised by some reviewers. I rarely shoot with the aperture fully open, and that's probably why. The S2 has its benefits, primarily that it's fairly fast, if you're into using auto and P modes, but the quality of the output using those two auto modes is pretty bad. So, to get a good result, you're back to the more manual shooting modes, like AV, TV or full M. The S2 also benefits from some tweaking, such as setting the EV to -1/3 and turning the sharpening to the lowest setting. It does pretty well like that, but it's still no G-series. The zoom is nice, if you need it, and the IS is needed because of both the zoom and the slower lens. The S2 is also plastic, which is an issue that doesn't bother some, but I can easily feel how much more solid and better-built the G-series are.
My point is that you're comparing apples to oranges here, and you've got to get your hands on a few cameras and see what you like and what you need. None of these cameras takes great pictures all on its own, but with an experienced user all can. I don't use the G5 much, and having both the G6 and S2 allows me to have two cameras that compliment each other, but if I could only keep one, it would be the G6, hands down. I hope some of that helped you.