Transport Convoys

Makes them faster, gives them more HP and higher A/D values. Unlike with other research, these benefits are "free": no need to pay for an upgrade. This is by far the most cost effective research you can invest in, assuming you're playing on a map that requires ocean crossings: World at War, Pacific Conquest, Historical World War. The speed bump alone is worth the price. Cutting a day of travel across an ocean can be game changing.

Especially if you’re an isolated country like Japan or Australia. Not to mention the HP increase. A strategy I use is spam L1 AT (I never research past L1 of those) put them in convoys, and add them to my naval stack. In the event of a naval battle, the convoys will serve as meat shields, keeping my battleships and cruisers alive longer.