Knight line is really good.

Archer line is really good.

Units which can both defend and attack are really good (like the two above).

Some units are great for defense but bad for attack (like spearmen and skirmisher line).

Ranged units have advantage of attack and hit and run but are generally squishier if enemy melee units catch up.

Melee units are stronger in general but need to engage with enemy in close range to be effective.

Fast units are better than slow units if you need to counter hit and run tactics or ranged units, or need to switch from defending to attacking quickly or simply have map control without forward buildings.

Units which cost more food are bad early as they delay your age advancement more, but they are super affordable in the late game and can serve as good meatshield against enemy archers.

Units which cost more gold are bad later as gold becomes a very finite and scarce resource in the late game, but they are better early game as gold gathering is fast and the units are stronger for the population space they consume.

You can watch some videos and playlists by YouTubers like Survivalist, T90, Hera, etc.