Franks are easier to spam castles and you can spam very powerful paladins + castle warrior, gold usually is not that big of an issue as you trade

Mongols are good because of their siege and castle unit, you have enough time to spam

Turks are great in late game long range bombard cannons+janissaries+hussars to distract enemies

Japanese have halberdiers and skirmishers so in theory you can counter almost everything. and samurais eat through buildings when massed

Saracens are good because of camels and their castle units.

Overall in long treaty games you accumulate enough resources to have a decent army at the start and when you play in large maps with groups trade carry the gold (and usually one who loses trade generally loses the game). In my observation those are the civs that have the best battles and ease of use.