I would have to say that the 30/30 is probably the most common hunting round.
I have never killed a deer with the 30/30, since I don't have one. I have lever guns but not the 30/30. I have killed deer with the 30.06 since I have one of those.
Now, all that being said, by most common, are you talking more deer killed with or more hunters usings. My guess is that more hunters use a 30/30, since many a dad will buy their son/daughter that gun first before stepping up to the 30.06 round.