Best method of packet transmission between an array of sensor nodes?
Best option would be either the onewire protocol or perhaps a half duplex UART.  With half-duplex UART, all you do is connect all of the UART RX and TX pins to the wire and just keep all of the TX pins tristated except for the one microcontroller that's transmitting.  You could run in to contention issues with the half-duplex UART method; onewire does not have this problem as it is open-drain.  You shouldn't have any issues with onewire on an attiny85, aside from perhaps a bulky library implementation eating up all the program memory.