DAC connection: which type gives highest fidelity?

You will have to listen for yourself. Every manufacturer has a different presentation with there inputs/outputs. Also, not all formats may be available on each output so you may not have a choice. You may need to use several types of interconnects to listen to various formats.

USB async would be the highest fidelity but for the fact that USB sources are often very noisy. Isolation is key and not all USB interfaces have that. Coax normally does though, but it'll be higher jitter than async USB.

The best is probably up for debate I2S, AES/EBU, etc. The worst, in my experience, is toslink. Your mileage may vary.