We use General Silicones 6mm2 Ultimate wire (Most Model shops sell this) for our heavy and I think they have 512 strands so are ultra flexible, making wiring alot easier but silicone isnt a very tough insulation but a soldering Iron wont melt it.

Be warned if you only use 6mm2 wire when drawing over 100Amps as the wires will get very hot, we use parrellel wiring from the 3 battery packs to the link to share the current but the wires need to be the same length to share the current equally.

It really all depends if you can tolerate the heat and power loss in thinner wiring, Search the internet for wire sizes and you can easily find charts giving temperature rise & power loss for different lengths and cross sectional area which will help you select the correct wire.