For a bit more detail, Babylon's Lancer spam can easily do most easy-mid lvl legendaries, and if your lancers are good you can do even the harder legendaries - The charge attack of the Lancers combined with being a fairly tanky cavalry unit makes them excellent at fighting infantry and range combinations, though if the enemy AI heavily uses anti-cav cavalry you'll want to mix in some mounted archers to help abate that. Keep in mind that Lancers aren't very good at taking out beefy buildings, so you'll want to keep a small contigent of either palitonins or Sappers depending on the map (Pals if you can, Sappers if the bonus objective is no pal/no siege. Generally 2-3 pals or 10-15 sappers are fine). You can use Neb to further buff Lancers, Chariot Rider Sargon to use the Chariot Archer or War Chariot builds, and finally General Zu enables the Royal Guard build, though keep in mind that Royal Guard's aren't great against heavy range.
For Celts, you actually have 4 different builds of varying viability - The most common is simple Woad Raider spam. Keep in mind that you'll want Brann the Bloodletter to make the most out of Woad builds. If you lack Brann, Champion+Druid is a solid build which doesn't require much to get rolling - Brennos is best, but the much cheaper Vercingtorix works as well. You'll want roughly 1 druid for every 3-4 champs in this combo. The 3rd build is Horseman+Carpetom via Commontorious - Horseman are used to tank and destroy buildings while Carpetoms kill everything. The final build is simple mass Bowman due to their high range, though it's also fairly gear dependent.
In terms of gear, if you go with Bowman/Chariot Archers, you'll want to maximize range for bow/arrow slot and damage(Crit on Bowman) on the armor slot. Lancers prefer build is Pharaoh's Spear of Destiny/Golden Hide of the Minotaur, though for the armor lesser armors with Bonus Damage Protection is good as well. Woad Raiders want to maximize damage on all 3 slots - Pure damage is best, don't listen to the goobers who say to max crit instead. Royal Guardsman want Apoc/Alexander's Breastplate as they already have a signifcant source of Bonus Damage protection from their advisor, either of the Vendor swords, and either Ares's War Shield or Shield of Rostam.
For Champions, the absolute BiS sword is Zahhak's Sword of the Undying due to Champion's high innate tankiness, though if you can't afford that the damage swords are solid. Due to the multiple HP buffs they can get, Delian Archon's Cuirass makes them the tankiest beating out Legion Enforcer's Breastplate that you use on other lesser medium armor units, but Apoc/Alex works too.
For Druids, you want maximum healing on their staff, and maximum health otherwise.
Horseman are geared the same as Lancers. For Carpetoms, you want Curse of the Archer along with damage cloth armor.