There are many great units in the game and I often only speak highly of heavy infantry and heavy cavalry, whom I consider to be bedrocks of the army, but I might have to reconsider that idea slightly. In my Makedonia campaign, I've finally reached northwestern Europe and have conquered northern Gaul. I fought against the Aedui, who recruited the Belgae Spearmen and used them in a few battles, but I didn't really notice how immensely bad-ass they were until I recruited and used them in my own armies. I didn't think they would make much of a difference, just like any other light infantry unit with no armor and nothing to protect them but a shield and their defense skill. Their stats are arguably misleading of how effective they really are in battle. This is indicated in ancient primary sources like Julius Caesar's De Bello Gallico, quoted in a loading screen of the game using the Latin "Horum omnium fortissimi sunt Belgae", or "of these, the Belgians are the bravest". You're goddamn right they are!
Here's a screenshot of their stats and appearance:
https://i.imgur.com/idz0R8l.jpg
In the very first battle in which I used them (ironically against Hayasdan, marching them to the northeastern front in the Pontic Steppe), I thought of them as a throwaway unit that would advance against either enemy infantry or cavalry at the bottleneck of a nomadic camp entrance. There, I planned to use heavy infantry and other skirmishers behind them to pelt the enemy with javelins while the Belgae Spearmen would just take on all the damage while defending the other units from attack and distracting the enemy. They did a bit more than that! They not only suffered way fewer casualties than I had expected and strongly held their ground, but as soon as the enemy buckled and ran in the opposite direction, my Belgae Spearmen quickly decimated them with their javelins and then ran at lightning speed to cut the rest of them down.
I was in pure shock. They ran after the fleeing enemy almost as fast as cavalry. The moment I understood their true potential I used the other three Belgae Spearmen to flank the enemy on the other side of the nomadic camp. I almost felt like some German soldier doing a Blitzkrieg against the Poles in WWII (not every day you feel like a Nazi). The battle was over before I knew it.
Just thought I'd point this out, and how excited I am to use them again in the near future. God they are bad-ass. You definitely made them perform according to Caesar's hype!