In your opinion, what is the most underrated military unit in the game?

I'll name a couple candidates that come to mind for me:

Infantry line, probably most specifically Longswords (not that Longswords are the strongest, but the most underrated stage) - To me, infantry use is situational, but the community acts as if it's completely non-viable. I've said this before but I feel if you're entering Castle Age at the same time as your opponent and you've already got map control and/or a good infantry count, then infantry are a cheaper, easier to produce alternative that can trade cost effectively with Knights thanks to your numbers advantage, as well as do a better job of pressuring production buildings. An added bonus is they share upgrades with pikes (cept of course the pikeman upgrade), so it's ridiculously easy to mix in a couple to make your dominance over knights even stronger. I think a lot of people would assume full pikes is better, but the infantry do a better job of countering trash (skirm-knight is a thing) and pressuring buildings, not to mention if a knight targets a Longsword, the kill takes long enough that a pikeman getting free hits in will destroy the knight.

I also feel like Longswords are an underrated counter to Eagles. People worry they'd be outrun by the Eagles so these aren't worth using. In reality though, you can sprinkle these around your base much like you would with Spearmen vs. Scouts and they put in tremendous work. The thing is, you don't need mobility vs. an all-in Eagle play. Let him push, because he's burning through his gold and if you hold off the push, he's 100% dead. Sprinkling these guys around defensive positions absolutely can suffice to outlast the push. I think another fear people have is Slingers or Jaguars being made in response. In my experience, Jaguars rarely come in a large enough mass to matter. Slingers however can be a real threat (they're effectively hand cannoneers) and need to be good micro to beat; you can easily counter with skirms, but now you have to ensure you micro your skirm-Longsword group better than they micro their Eagle-Slingers, which can easily go either way.

Is it the most versatile member of the rock-paper-scissors game? No. (unless maybe if you Malians, those guys are great) Does it however have a use that is heavily neglected by the community? Yes.

Teutonic Knight - I don't think this is just a meme unit. To me, this is a unit you make a limited supply of and defend with. Fill a castle with these bastards and they're tanky enough to run down anything that tries to attack before retreating to the castle again for heals. The bad reputation comes from people going overkill and making too many of these, but if you limit yourself to 10-20 of them, they're fantastic.

These take 1 damage from all trash units and therefore need 100 hits to kill when facing trash, they practically force an archer response from your opponent, meaning you can then run wild with skirms or cav, even vs. xbows they make for respectable tanks (2 pierce armor and a minimum of 80 HP), and even a humble squad of 15-20 can be a serious threat to the enemy base, as they burn through buildings.

Basically, I think this is a unit that should be made in modest numbers to force certain responses from your opponent; it's purpose isn't to single-handedly fight the opposing army, but rather to DEMAND a response that the rest of your army counters well, where if the opponent refuses to respond to the Teutonic Knights with a counter, now they're a serious threat. If you have a forward castle as Teutons and fill it with these guys, he specifically needs trebs + arbs (or CA) to stop them from running down the trebs before returning to the forward hold. If he tries legit any other unit combination, these guys can beat them OR trade cost effectively. Likewise, if a raid hits your base and you have a forward castle filled with this, just let them out and he'll likely have a far more difficult time stopping your squad of Teutonic Knights unless he's raiding you with 20+ Paladins or something.

It's also a unit you should 100% heal, and I'd argue Teutonic Monks specifically deserve their own spot on this list, as their extra heal range + the high gold cost of the Teutonic army means they both do well at healing from a safe range and they highly benefit the Teuton's army composition.

GBETO MASTERRACE

Are your infantry losing to opposing infantry? Not with a bunch of knife wielding strong independent black women behind them, they're not. Do you need a unit that tears through buildings? Gbeto. Do you need one of the fastest villager killers in the game? Gbeto. Do you want a unit that looks at a mangonel counter and says "no u?" Gbeto. Do you want a unit that, even when faced with a cavalry counter, can trade cost-efficiently and make sure those cavalry bleed? Gbeto. Do you want a unit that, even with a humble squad of just 4 units, can seriously slaughter the enemy villagers? Gbeto.

Seriously the best damned unit in the game. The only scenarios I can name where there's limited value to having them is if you're forced to face a high number elite skirm army (trades inefficiently) or a high supply of Onagers. (a small amount can be micro'ed, a large amount is a serious gamble, especially when Farimba Light Cav do that job more safely)

Gbeto are such a valuable flank unit that can either drastically improve the effectiveness of your main force (deals heavy damage from a safe range or forces the opponent to waste time just trying to catch the damned things whilst the other units of your army get in free hits) or provides an on-the-fly raid as a counter response to any pressure you're being faced with. Unit is already respected as being good, but that's not good enough! This unit should be considered the queen of AOE 2.