Warband, easily.
Though, tbh, if you get the Native Expansion Mod, original might be a bit better. Warband is basically an updated, more polished version of the original with better textures/animations and an additional faction.
Native Expansion, though, does a great job with the balancing that even Warband kind of fails at.
If you're not going to get Native Expansion, make sure to get the TweakMB mod. It's essentially a simple editing tool that lets you modify some numbers in the game. First thing to do it increase the number of bandits (except Sea Raiders, Steppe and Desert Bandits). Otherwise, the game world quickly becomes barren except for the war parties that you can't take on until much later in the game.
I haven't tried Fire and Steel yet, but the reception it got seems lukewarm at best.