Cephme
Chieftain
It irritated me in Civ V that the Submarine was the first available modern naval unit. Looks like more of the same in Civ VI.
I found I almost always got Destroyers before subs. Maybe it was just the tree I selected. Are you saying it is the easiest to rush? Also the Eureka for subs requires 3 privateers (which I am guessing upgrade to subs now instead of destroyers which is cool and gives them a more unique feel than they did in civ V vs frigates), which we are thinking requires mercantilism, a civ not a tech, so requires a completely different branch. I think the cross over eurekas are going to be harder to groc and get when figuring the fastest path to a tech.
FYI Subs (1885) were deployed before destroyers (1892) as we know them so honestly subs coming first is probably right.
https://en.wikipedia.org/wiki/Submarine#20th_century_submarines (see Nordenfelt I)
https://en.wikipedia.org/wiki/Destroyer#Development_of_the_modern_destroyer (see Daring Class)