One of my favourite factions to play in R2 is Pergamon, but it is DLC.

You have insanely high charge elite cav generals who don't have the tankiness of Cataphracts. Early game charges are brutal, and it is not hard to gain buckets of experience for your generals and claim the 'Claret' traits to drop enemy morale. However, you still need to be aware of risks to your general in combat.

Their late game roster has some really nice units, like Picked Peltasts and Agema Spears, which are amazing in their square formation.

They have a nice challenging early game Galatian roster Iirc, but but have easy access to Merc Rodian Slings and are close to being able to get Syrian Archers, and have challenges on the doorstep like Pontus, Seleucids, Galatia, and Macedon, while being able to Client State recruit from Colchis or Sparta to claim more conventional disgustingly powerful Hoplites if you push for that and economically support them while you head south and east.

Like all games, it can become a Steamroll and make end game boring, but they are one of the few factions I've been able to enjoy to enjoy through to that stage, and claim the campaign victories in legendary, as well as military and economic victory.

One thing that also might seem a bit daff, is actually roleplaying along with the missions you are issued or bonuses you receive, and trying to complete even if it seems like more effort, or it is something bad might happen. Or the old chestnut of recruiting a full stack for a campaign, only to then get negative recruitment cost or veterancy modifiers for your elite units the following turn: instead, you only recruit those units when you have the cost modifiers.

It adds to challenge.

The other thing I think should be picked up is an agent mod. Rome 2 agent spam is especially bad IMHO.