The two best models would be a IIa fixed lens 50mm f/2 Xenon lens or a IIIc/C with 50mm f/2 Xenon. The Xenar 50mm f/2.8 of the IIc is a fine lens but the Xenar's are best.

The IIIc/C models have interchangable front componant lens but can be a bit confusing except for the 50mm..

My personal favorite is the IIa..