In your opinion, what is the most underrated civilization?

I'll start:

I think Vietnamese are a solid archer civ that doesn't quite fit into the meta strategy of archer civs, so they end up with a lower winrate and lower popularity. It's true they have flaws that need to be overcome, but I also feel like they're better than thought and that much of those flaws come from Vietnamese not quite fitting in with standard archer strats.

The thing is, the Vietnamese wood savings enables you to get all the early eco bonuses without disturbing your archer production/farm production rate at all. Since we tend to only get Horse Collar and Double-Bit Axe for Feudal, this is a modest bonus that helps your eco moreso than your actual archers. The health bonus is very humble in Feudal and does little to make a difference, admittedly. (tanks one additional shot from enemy archers for example, but this matters little when overkill is common due to micro'ing entire archer squads to attack one unit)

I think their real power spike is actually castle age, where they can still support their boom easily thanks to the lack of wood costs on eco upgrades. Their eco bonus is often superior to the wood savings Britons enjoy, just doesn't necessarily feel like it since Heavy Plow + Bow Saw is 225 wood saved once castle is hit whilst Britons save 275 wood on two TCs, but this doesn't count savings made already in Feudal or the greater potential to grab Gold mining early. What the savings on wood via eco upgrades instead of TCs does set you up for is an aggressive castle age backed by only 1-2 TCs, though.

What I like about Vietnamese is that they set you up to mass archers, then they have Battle Elephants available. The typical counter to Battle Elephants is Monks, but Monks don't do so hot against archers, so Vietnam in particular has a really solid mix of elephants - usually ignored due to their heavy weakness to monks - combined with a unit that can prevent that major weakness. I actually think Vietnamese played solely for archers is, understandably, a very underwhelming experience when you consider the advantages that civs like Ethiopians and Britons provide. What Vietnamese offer though, imo, is a better combined composition that works well together, though it is trickier to pull off. Where other civs would likely go into Mangonel micro wars, Vietnamese can likewise just actively tank shots with elephants that exhibit pressure on buildings, enemy mangonels and more. Even if the elephants are dying, they tank enough damage for Vietnamese to pull ahead on archer numbers in archer mirror fights. Battle elephants have the added benefit of killing buildings faster than Mangonels, being producable immediately upon hitting castle instead of needing a new building, and a smaller gold cost. The building damage in particular is nice since attacking buildings is usually the major weakness for archer raids.

I understand why this still has a flaw with speed though, and I think that's part of why Vietnamese underperform: I think their most potent gameplan relies on breaking the mold and going for an archer-cav mix, and likewise securing forward presence - but not necessarily forward aggression - in Feudal. Their Feudal is fine, but doesn't exploit their bonuses as well as an early castle push. You want to grab a forward position to push hard with in Castle Age and reduce the walking time for your T H I C C horses.

Finally, by the time they have a Castle, they're immediately far more threatening vs. archer civs, and their imp unit composition is actually one of the most flexible, only lacking Blast Furnace, Hussar (not as big of a deal as people think; it's basically 15 less HP, but when using a civ that can support it's Light Cav with good ranged damage, it's not as big a loss), the second siege upgrade for all respective siege weapons (they get BBC though) and Paladin.

Basically, it's a civ with the right bonuses to set you up with a healthy eco and a healthy archer mass by the time you hit Castle, and by then it only requires you to capitalize on their advantages via aggression. Once their desired army is created, it's an army that takes forever for the opponent to weed out, allowing you to create some serious idle time within the opponent's eco.

I think the points against them have more to do with demanding players break the mold of pure archer spam, or difficulty with getting just the right amount of aggression going around Feudal so that you can create forward stables for a solid push. It's admittedly a trickier civ to excel with than Britons going "HURRDURR RANGE" for that reason, but it's likewise a civ that I'm often surprised to see rated as low as they are.

Also they got really sweet hats.