As far as the raid itself goes, Helios is definitely the best (coolest). The problem is that the difficulty is skewed so outrageously between each raid. Dojo is effortless. Grab a few mechs and set it to auto. City Hall is slightly more challenging, but you can still set it to auto and forget about it. Helios on the other hand is drastically more difficult than anything else out there. Obviously at the top level, everyone is just throwing it on auto anyway, but the utter lack of difficulty balancing across the board for co-ops is pretty shocking.