Side by side comparison by total research required.
Tier 2
Tier 3
Tier 4
Comments
Melee:
Melee units are often considered the default thing your army consists of. They're the most expensive to research but the research path always includes access to metals for their weapon upgrades. The same technologies are also requirements for many others and there are several good opportunities to switch to other research paths and unit types. Bronze Working is never wasted in the long run.
Immortals seem really really hard to reach but you should understand that Divine Essence is priced as an extra to consider when you're getting mithril upgrades and High Priests anyway. Same for the Phalanx tech, just going Mithril Working to upgrade your Champions (for example) is good too.
Archery:
Archery line has medium costs to research. If Mining shows no copper, Archery tech looks immediately more attractive for early defense. The lumber mills and 50% chopping production bonus may also suit your start. T3 requires Bronze Working anyway so going Axemen/Swordsmen first is ok too. Alternatively your ex-Warrior Archers could develop to Horse Archers.
Archery units are well suited for defensive strikes and first strike upgrades, so it's good to have some along with your offensive stacks if you have access to the units. Longbowmen and Crossbowmen get weapon upgrades, Marksmen get their namesake promotion without the city attack penalty of assassins. The later tiers end up viable for massed offense even if you aren't playing Ljosalfar. Arquebusiers are the only T4 unit that isn't limited to 4 per nation. Note their synergy with the earlier tiers of the Archery line isn't great, for example teching them after Champions is more viable. Getting both them and Crossbowmen works nicely though.
Mounted:
Medium to cheaply priced tech-wise, Mounted units are best suited for offense or perhaps aggressive defense. Unless you're Kuriotates, they won't have defensive bonuses from terrain and you'll need specific resources (usually horses) to build any Horsemen at all. Hippus get horses from their palace, but others normally go for other T2 units first.
At T3 these units really shine. Horse Archers are very fast to research! Stirrups itself is a dead end tech though, so make sure you take advantage of them while there's a good window of opportunity and your neighbors remain unprepared. Alternatively, Chariots can get weapon upgrades and their research path consists mostly of good economy techs. Chariots with or after Bronze Working should still reach the battlefront clearly ahead of Champions.
Civilization choice matters a lot when considering fast Mounted offense or perhaps hit and run raiding. Beyond Kuriotates and Hippus, Malakim and Svartalfar can always rely on fielding their Horse Archer replacements (on camels and felines respectively) because they don't require finding resources.
Recon:
Recon line is cheapest to gain access to overall, but also consists of dedicated techs you'd probably ignore otherwise unless planning to get druids. Recon units are strong in absence of metals, and fastest after Mounted. They can defend your lands or take you all the way to the gates of enemy cities, but someone else should siege them or pillage the surrounding countryside.
Hunters, Rangers and Beastmasters are the basic Recon progression. Assassins are high utility and quickly reached but tied to a dead end tech. Shadows don't have any special research synergy here but can be combined with any kind of army. If Esus spreads to your lands for free and you're already up to Code of Laws and/or Writing anyway, they're an even cheaper T4 unit to research and the path goes through economy techs.
Arcane:
After Adepts, Arcane units become a fairly expensive research subject but they can do lots of different things for you depending on your preferred mana nodes. Adepts are usually best at supplementing other units while Mages more often manage on their own or with a little support. Archmages open lots of funny options.
Mind if I use put this on the FF Wiki?
