Best test subject for *isDS JPEG compression testing?

[Glen]

Does anyone have suggestions for an "ideal test subject" that I can use to 
evaluate how much compression I can tolerate in my JPEG files? Sometimes I 
shoot in JPEG mode to get more exposures on my card. A lot of the time, I 
can't tell any significant different difference between different JPEG 
compression modes. What type of subject would highlight the various JPEG 
flaws and limitations the most?

thanks,
Glen