Green Tech Tree/ Azure Dragons should always be your first unlock.

[WildArmsFx]

I've played four campaigns with pretty different factions all on the hardest settings. The bandit queen, cao cao twice, and Kong Rong. I cant think of any reason to not start off unlocking most of green tech tree down to Azure Dragons. Even with an heavy industry eco or commerce based eco, green is still the best to unlock 1st. I think there are 3 or 4 individual ones worth grabbing like slave mobilization and few buildings. Even with Kong Rong, a commerce and trade based faction, green tech tree is best. After unlocking the next path should be final Admin building thats a combo of the red and gold path.

Cao Cao which seems to be the most popular hugely benefits from green tech tree due the fact he starts in the farm region and already has highend shock cav unlocked.

Peasents are going to the base and majority of your eco in this game with food directly linked to growth and expansion. Even with the whole tree unlocked and every 80% of cities having land development and government support I struggle to feed my Imperial Cities without using valuable Champions for food assignments. So its not optional like other chains.

Eco-
A peasant eco is the best. It just doesnt "auto-trade" like commerce and requires you the player to do so. I can sometime can 1,000's a turn by trading only 20-25 food early-mid game. Especially with Kong Rong and Cao Cao who can build good relationships. You should be the bread basket of your three kingdoms as its highly valuable to change relationships via trade. Give a hostile but not at war faction 10 turns of 15 food during winter and its will boost you relationship into the positive nearly immediatly. I think 15+ positive on a deal give you like 75+ positive relationship WITHOUT upsetting other factiona like alliance or military access would. Food is incredibly valuable to trade up to 1.5+ in a negotion.

Two building that generate more money then any other are land development and government support. With the bonus those two building can supply up to 30+ food (by boosting farms and land development) and on their own without farms in the settlment 15 food and 300-400 in income. Every play-thru selling food is a huge part of my eco. Even though it doesnt say "income" it produces something equal as valuable.

Building a commerce/industry eco without unlocking buildings till mid-game is better then high-end building on a smaller city. You're far better off get your city to 6 building spots with low tier buildings then trying to optimize early on with high level buildings. Military wise it makes things alot easier and its eco is as good or better. Until you have 6 building spots and a admin building fully upgraded to effect adjecent settlments, dont worry about high end commerce and insdustry buildings.

Military-

Green is by far the best Military Chain, even better then the actual military chain. Because it unlocks your defensefor almost all you commandries upgrades giving you large and costless garrisons. Mines are upgraded in a seperate path but only take 1 reform to unlock. Trading ports are the same and require a single upgrade point in a commerce tech, but arent worth it because it has junky units and will most likely need a general to defend. So besides a single reform it ups all your defense, really good early game as you can defend with one a single retinue or cause enough damage to severly weaken the enemy unit.

It also allows you to upgrade cities for better defenses by generating more food. Military Garrison building chain does not require you to unlock a reform to get its full retinue. So with an admin you can easily defend a city with only a single retinue army. Especially if you max-out your admin with best units available, its good because you dont pay upkeep for them per turn but they are always there to defend.

Azure Dragons are the best late game unit, better then gaudrians of the earth. The AI really does not know how to deal with them. The mixed missile formation is so deadly on defense. Something you can force every open field battle with patience and catapults, even if you are the one attacking They also allow you to have crossbows with no bow archers because they can wield fire arrows. With Kong Rong they are unstoppable because of his upgraded crossbows. I really think the devs need to give the AI a better direct count for them or nerf the mixed missile position to receive more damage from ranged attack and less damage reflected against mounted. I wrote a pretty in depth thread using them as the core unit of my army. I've beaten 4 armies against the strongest faction in a late game battle on the hardest setting. No other unit is as powerful with unique advantage. They arent even that expensive compared to other late game units like emperor units.