My personal favorite is the legionary cohort, for all the reasons mentioned. Rome was historically infantry heavy anyway, and I usually play like that. My usual "field army" consists of 10 legionary cohorts, 4 archer auxilia, 2 roman cavalry and a general. I'll add more as needed depending on the foe I'm facing.
As to the engines, I've recently started using scorpions in certain battles, mainly because in my current campaign Armenia has become quite powerful (they took over the Parthian starting lands, the city of Seleucia, and two of the Pontic provinces.) So, now I'm up against a mix of crap infantry, decent infantry (heavy spearmen and armenian legionaires), crap missile troops, and gods on horses. My usual army wipes the field with all the infantry, but those cataphracts cause horrendous casualties whenever they get into combat. I hired a couple of units of merc hoplites, and had one of them run over by a unit of cataphracts (not generals bodyguards, just the normal cats). Scorpions have a longer range than either the ballista or repeating ballista, and does enough damage that if you manage to hit the unit, you'll kill 1-4 people with every bolt. It doesn't matter how armored they are, they die. Once the unit gets down to only 1-2 ranks deep it becomes much harder to hit, but until then you hit them regularly. I haven't had the chance to use repeating ballistae yet; I just finished building a couple of them but they haven't made it to the battle area yet.
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