Knights are generally a good raw power unit in the early-late castle age. The only units that counter them 1 on 1 are Camels, Teutonic Knights, War Elephants and monks (if guarded). Their Paladin upgrade is expensive (1300 food, 750 gold) and time consuming  to research. Paladins are the most powerful land unit with a generic Paladin having 180 HP, 18 attack, 5 hack/ 7 pierce armor. Only a few civilizations have access to the Paladins and they are: Spanish, Byzantines, Celts, Magyars, Persians, Huns, Teutons, Lithuanians, Cumans and Franks

Celts lack plate barding armor and bloodlines. So, Celtic paladins actually suck late game.
Byzantines lack blast furnace and bloodlines. So, these are not good either.
Byzantine paladins>Celtic paladins as armor is more important for melee units.

Cavaliers give 20% improvement to Knights (in terms of attack and HP). So, try to have at least 20-30 Knights before getting the Cavalier upgrade (for the power spike) and at least 40-50 Cavaliers before getting the Paladin upgrade.

You should invest heavily in the Knight line only with civilizations that have plate barding armor, blast furnace and bloodlines to get the most out of your resources

Civilizations that should invest in Paladins:
    1. Huns: No need to build houses, 20% faster stables (team bonus), Cheaper Cavalry Archers to counter halberdiers
    2. Spanish: Blacksmith upgrades that don’t cost gold, faster working builders to build more stables and a team bonus to produce 33% (25% in the expansions) more gold to help mass an expensive unit.Late game, it is the gold that matters for Paladins. So, a trade cart that produces ~57 gold in a trip would produce around 75 gold (which is the exact amount of gold needed for a paladin). This number has been nerfed to 60 gold being upgraded to 75 gold in the expansions. Also, the fact that fewer trade carts are needed for the same gold means your entire team can have more paladins.They also got faster firing Hand cannoners to counter their main weakness (halberdiers).
    3. Franks: 20% extra HP, Chivalry tech (40% faster stables) in the expansions, +2 Line of sight (team bonus), 15% faster foragers in the expansions make it the best civilization for Paladins. 40% faster stables after Chivalry makes your stables outperform even Hun stables. This also means you can get the time consuming Paladin upgrade much faster and spam these units like crazy.
    4. Magyars: Free attack upgrades (forging, iron casting, blast furnace) makes for a decent power spike as you age up. This affects the Knight line too. So, the Magyars can do a decent Knight rush.
      Their Cavalry Archers benefit from the Recurve bow tech which gives them +1 range and +1 attack.
    5. Persians: +2 attack bonus vs. foot archers as a team bonus.
      Not huge but decent.
    6. Teutons: They lack husbandry but this is not a big deal. They get conversion resistance as a team bonus which tends to mitigate a weakness for the knight line (monks). With a recent patch for the definitive edition (UPDATE 36906), the Teutons now get +1 melee armor for each age from Castle age.. making it +2 for Imperial Age. This applies to both Barracks and Stable Units. This would help a Knight rush for the Teutons and even provide them some raw power in the Late Imperial Age as their Paladins would have 7/7 armor.
    7. Lithuanians: Their Knightline and Leitis (previously ALL cavalry) receive +1 attack for each relic garrisoned for a maximum of +4 attack. Previously, this was capped at +5 and applies to the scout line as well. With 4 relics along all Stable and Blacksmith upgrades, Lithuanians have the strongest Paladins in the game. Their attack is capped at a mighty 22 in the Post Imperial Age.  They also start with +150 food which helps their early game economy a bit. Fun fact: A Lithuanian Paladin with full relic benefit can even beat a Frankish Paladin. However, this bonus requires the Lithuanians to have relics garrisoned in their monasteries to get the benefit. So, it’s a little situational unlike the bonuses of the Franks.
    8. Cumans: Cumans used to get 10% faster cavalry right from the Feudal age and they also had access to Husbandry.. making it 20% faster cavalry. Now, it has been staggered at 5%, 10%, 15% faster at Feudal, Castle and Imperial Ages respectively and Husbandry has been removed. This helps them with a Knight rush and also helps their Paladins rush in Deathmatch games.

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The Knight line has the following weaknesses: Monks, Camels, Spear line and Mamelukes. So, massing paladins in the following situations is a bad idea.

  1. Against Monks especially if they’re Aztec.
  2. Against an army of the Spear line (especially if Japanese or Slavs). Halberdiers lose 1 on 1 but they cost no gold.
  3. Other Anti-cavalry units such as Camels, Mamelukes and Kamayuks.

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Civilizations that don’t get Paladins but have a bonus or tech that directly helps the Knight line:

Bulgarians: All cavalry units (including their Knight line) benefit from Stirrups tech which gives them good knights in the Castle Age and decent Cavaliers in the Imperial age. However, they still lack Paladins.

Malians: Their Knight line benefits from Farimba unique tech. This gives them 17 attack Cavaliers which are pretty decent. However, they still lack Paladins.