U-124 - the Edelweiss boat, commanded by remarkable commanders and knight's cross winners Georg-Wilhelm Schultz and later Johann Mohr; this is largely thanks to the book "Grey Wolf, Grey Sea" (highly recommended!).
She was the 4th most successful boat of the war (and behind the 3rd most successful, U-123, by a mere 748t).
It would seem that her loss on April 2nd, 1943, should largely be attributed to Mohr's tragic underestimation of allied radar technology (which, combined with his daring surface tactics, ended up in a grave miscalculation on his part...)