Upgrades! I'll mention some neat end-game Supremacy ones which i tried and found very good.
1. +50% flanking, +20% near-friendly-unit Redeemers are extremely efficient:
- fast (4 tiles per turn, no terrain costs, hovering means they are also, for all most intents and purposes, "melee naval ships" when at sea - except they can _heal_, unlike proper ships, outside of friendly territory when fortified, and they can _fortify_ while at sea),
- expendable (no resource cost, and relatively cheap),
- can take cities all on their own,
- can take the same tile with a carrier, effectively protecting the carrier from harm with their much higher combat power,
- are excellent at pillaging tiles, trade route plundering and capturing "panicking" workers.
- the +20%-near-friendly-unit bonus works both for offense and defense, and +50% (total) flanking works for any attack made by Redeemer whenever there is at least one more friendly unit adjucent to the Redeemer's target, and also during defense against any _melee_ attack whenever attacker has at least one more friendly unit (except the Redeemer) adjucent to the attacker, afaict.
2. "Can move after attack" Vindicator. It's quite simply _the_ upgrade which allows you to dominate an ocean even with 1-2 Vindicators. Being able to move out of harm's way after an attack is amazingly useful. Once, i have a lone Vindicator fighting 7 (!) enemy (AI) Vindicators which were upraded for +1 aircraft. I "shot and ran" them all, one by one, going in circles in a large ocean... %)
3. The Supremacy upgrade for Tacjets which allows 1 free rebase per turn - if you enjoy having any large air force, this is surprisingly effective.