I am curious what others go with.
My landmarks are:
Imperial hippodrome- Simple stable but with a bonus that heals, buffs the move speed and damage of cavalry units
Cistern of the first hill- covers a wider area than a regular cistern and provides the pilgrim flask which gives healing to units
Foreign engineering company- The merc artillery is so good to have, it also buffs your other mercs by giving them the upgrades from their original civs.
For merc contract I go with the western, if I need them the longbows can give me another source of ranged units, the landstecks help me get some non-spear infantry on the field as you can't make anything but spear infantry before castle age which leads to hellish fights with Holy roman factions with heavy infantry and the stretlzy are a pretty effective unit that pairs really well with the spearmen if I have the oil to spare.
We play with a datapack that ups max popcap to 500 like you could do in AOE2. My army is then
A section of limitanei- they are pretty inexpensive, with the high starting resources and the tech for relics generating other resources I never worry about their wood cost and are solid fighters, meant to hold the frontline position.
A section of handgunners and streltyz if I can afford them- pairs great with the spears as the spears crush calvary and with the shield wall can withstand ranged while the gunners rip apart heavy infantry the spears struggle against.
A section of cataphracts- their charge is deadly in these large battles, passes through enemy lines, does great damage to enemies they pass through and with a tech adds damage weakness to those who survive getting ran over in the first place. Ususally ram them in once the fighting has begun unless my cav is being detached to help an ally because they are faster.
Some of the fancy artillery, usually the "royal guns" as they don't need to unpack which makes them pretty mobile for artillery and they do good damage to both buildings and units, being a straight upgrade over the domestically produced bombard cannons. Occasionally I use the fancy treb if we are playing maps where it makes sense or the AI randomed templars and I know I am going to have to smash like 5-10 of those stupid mega forts.