Best test of turretless TD's I can think of would be to get the SdKfz 251/22, as far as time spent on a potential failure goes. Purely because they don't have to model the casemate on the Jagdpanzer IV.
Just an SdKfz 251 with a PaK 40 mounted on the top.
If time/effort spent is of less concern, I would say Jagdpanzer IV, or Jagdpanther is the best we could do. I would vote for the Jagdpanther, purely because even if it ends up being less usefull than anticipated, it could still have a role as a dedicated Tiger-buster with its 88mm L'71.