In P2PKH Multisig Transactions, what is the address?
There are no "addresses" in protocol. There are only "output scripts". The address is only human-readable representation of script which matches the "output script".

The output script for bare multisig transaction is [m] + [hash1] + [hash2] + ... + [hashN] + [n] 

And all of these parts are significant. There is no "classic" representation of such address yet.

bc.i uses the form of (Escrow X of X1 X2 X3) (where Xn is standard p2pkh address) but you may use your own.