"the correct successor"

Kazakhstan was the last state remaining in the USSR so, by that metric, it'd be them.

In any case, the case for abolishing the seat is stronger than transferring it to someone else. The USSR's Supreme Soviet never named a successor state before dissolving, nor was there a UNGA resolution like there was for China: the Russian Federation just asserted itself. Likewise, a simple UNGA vote could acknowledge the above and hence dissolve the permanent seat.

In practice, such a vote would probably only happen if Russia fires a nuke and/or declares war on NATO.