A lot of people are saying Kurama, but that's due to Kurama being dramatically more powerful than the other
tailed beasts. However, under the OP's premise, with all equal levels of chakra, Kurama shouldn't be able to beat
down the other tailed beasts so easily. The roar should easily be countered by another being of the same chakra
amount.
Thus, they should all be equally powerful. So now we have to account for special abilities and skills. Kurama seems
the most skilled, but its special quality is how extra powerful it is, while the other bijuu have special techniques.
Thus, I would say probably 5 tails, since it can magnify its own strength to topple the other bijuus.