I have both the RD and G-III. They're both excellent cameras - the Canonet feels fantastic in the hands, very solid and hefty. The RD is almost the same size as the RC with a hood, and as stated before, has an almost silent shutter and lovely winding action.
Personally, I like them both. The Canonet feels a little better to me, but the RD is stunningly featured for the size. I believe the RD also has a better lens.
However, the RD does have the sticky shutter problem. And it's a hassle to get fixed. The Canonets tend to be more reliable.
As for the SP, I don't personally know. They do have metered manual, which is nice, but their auto mode is fully auto - no control over either aperture or shutter. Personally, I prefer the shutter priority shooting with the RC, RD and G-III. You get a little more feedback but it's all automated for you.