Khergit, because there's nothing quite like hurtling across the plains (especially with GE 2.5 and RCM) rhythmically bring death with every arrow, lance strike or saber slash. There's a dizzying sense of freedom playing Khergit.
For infantry/archers I would say Rhodok, because of the good mix of spear, shield and bow in the regular, more expendable troops. If you want something harder hitting there's either the Noble or it's easy to recruit a few troops from another faction to fill that role. No point feeding good meat into the grinder.