Ferdinand/Elefant. Post-Kursk deployment, once some of the mechanical issues had been worked on and the MG installed. Huge gun, tons of armor, and... well, just pretty badass.
According to wikipedia (hate using it, but it is easiest)(
http://en.wikipedia.org/wiki/Elefant), "In terms of kills per loss, the Ferdinand/Elefant might well have been the most successful tank-destroyer employed during the war, reaching an average ratio of approximately 10:1."
Accurate? Seems possible, if not likely. Though mostly used in defensive/semi-stationary roles for obvious reasons- so this would definitely help the kill/loss ratio. Easier to knock out enemy vehicles when one just has to sit in a good firing position and shoot from long range.
Honorable mention- Hetzer. Possibly the best "bang for your buck" afv the germans produced.
