I want to control multiple LED panels that have each a chap ATTiny on them which controls some WS2812 LEDs. There's a Master controller with an ESP8266 that is meant to send commands to the individual panels like Panel1: color=red. Which would be the best solution to let the MCUs talk to each other? What i thought about:switches
1- SPI: I wouldn't use it, since I'd need a CC/CS line to every Panel
2- I2C: Better, Bus system looks good but the cheapest ATTinys don't support it
3- Serial: Not a Bus system
Some Bus system like CAN would be ideal, however this isn't supported on the smaller ATTinys and cost is an issue here.
Why not just put an ESP-01 on each panel? 
You will need a level converter - a 74HCT14 - on each to drive the 5 V WS2812s from the 3.3 V ESP.
But you could use it like a bus system. If the master sends a message, which all slaves receive, and the message indicates panel 1 should be red, then only panel 1 will respond and all other panels will ignore, because that's what their code says they should do. You also need to give each panel its identity in some way, of course.
But why are you adding an attiny to each panel? You don't need one to control the ws2812 LEDs, they can be controlled directly by the master.
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You can assign them each an address and send the same data stream to all of them. Each display will ignore messages sent to the other displays.
A "broadcast" address would let you set/clear all of the displays at once.
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