Of the ones that I have grown, Dixielee has the darkest red flesh. Color of flesh is not an indicator of taste however.
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LOOKING FOR MOST HEALTHY.I LIKE ALL WATERMELONS BUT PREFER THE DARKEST ONES.HOW ABOUT THE DARKEST TOMATO?

Darkest tomato would be one of the blacks. I don't like them so I don't grow them, but they are popular in many quarters.

It sounds like you are interested in lycopene, as in this USDA study.

The darkest watermelons, such as Dixielee, have the scarlet gene (Scr), but apparently the USDA research used standard red watermelons, so it is not clear if the scarlet red color is related to lycopene content. Watermelon Genes