What's the best internal SoC bus?
Definitely AMBA. 

Wishbone has synchronous writes and a-synchronous reads whilst AMBA has both synchronous. In an SOC design you want everything to be synchronous.

On Wishbone you continuously have to add in a wait cycle for the data to come back from a register or from memory*, whilst in AMBA the data flows without waiting.

The write cycle in AMBA is less adapted but:

1/ You read a lot more then you write in SOCs

2/ You can delay/store a write address, but you can't predict read data.

Between AXI/AMBA, AMBA the simplest to implement. AXI is nasty as it looks so simple but is difficult to get right. 

*All on-chip and on FPGA memories are synchronous these days.