Leningrad.
From a morale standpoint for both nations, it was the next biggest after Moscow. Due to its proximity and the option of flanking through Finland, it would have been the easiest to take. From there, other Lend-Lease insertion points could be cut off or captured.
Also, this would allow a unified front. If (as they did) the germans drove all the way out to Stalingrad, that army group is a major proruption from the main front line, which is easily cut off. From Leningrad, the entire army is mutually supporting and can advance together on moscow.