Most insufferable faction?
I've found myself giving the remastered a shot, the Julii, very hard. I'm mostly a seasoned player and I thought it will be fun to face the hordes of dumb barbarians as I slowly conquer and civilize them in the name of the republic.
Gauls were ok, it was fun to fight their stacks of warbands with my hastatis, equites, velites, some principes later on. It was tough but fair.
Iberians started to randomly desant on the italia and stirred shit up, and while it was a bit annoying, it was manageable and provided further to the challenge.
Then, here they come, the Britons. The Britons and their fucking stupid chariots.
Then they're attacking me NON-STOP. While I'm managing to hold and catch up with the rest slowly, I still need to manually fight on average 1 big battle per turn, and it's not fun. How they've gotten the chosen swordsmen so early?! And don't get me started on the chariots. 5 hit points??? With all the rest, fucking really???
They also destroyed Germans early and landslide across the northern Europe, which sucks as I had thoughts of fighting Germans and their berserkers in the late game and doing some Teutoburg forest type of fuck-ups. But no, instead I have to deal with a bunch of British heavy chariots for approximately next 60-70 turns : ).
Have I mentioned that I fucking hate chariots?