If all factions behaved with no bonuses, I'd fear Gondor once they got going.
I think the only faction as which I wouldn't fear a human player is as Isengard (when I play as Rohan)
You can blitz the out of him before he has any money to do anything.
Spoiler Alert, click show to read:
I have to go with HE and/or FPoE/Arnor.
Both of these factions are in perfect positions to focus almost all of their energy into building up an economy in the early-game. They only have 3 enemies in their vicinity who can attack them within the first 10-20 turns, all of which are extremely weak or busy being fought on another front (OoM, OoG, Isengard/Dunland (MoS)). Their late-game armies are arguably the strongest in the game as far as unit strength and composition compared to who they will likely be facing at that point, and they are so distant from the strongest Evil factions (Mordor, Harad, Rhun) that they can sit back behind their remaining allies and just send huge amounts doom-stacks while raking in the cash and still have enough of a standing army left-over to be able to assist against invasions.