At emperor or lower i would say without a doubt it is the iroquois. I have some HoF games to demonstrate this (all played earlier in my civ career, and i made many mistakes).
Iroquois advantage:
1] UU is Mounted Warrior !! - 3 attack, 2 move, for only 30 shields (only better conquest UU in Ancient Era is gallic swordsman, but the extra defense is mostly useless, and they cost an additional 40 shields.)
2] They begin with Alphabet, allowing the fastest most direct line to a better gov't.
3] The Agricultural trait is a must.
a general blueprint:
research: Writing -> CoL -> Philo -> Republic -> try trading for WC, and The Wheel -> then Horseback Riding -> trade for CB -> research Mysticism -> Polytheism. If you have chosen your opponents correctly you can stop research at this point (unless you feel the need for an SGL). Now you will have alot of gold for rushing mounted warriors and settlers.
place cities as far apart as possible, account for the space filled in after culture growth. you MUST build the ToA, below monarch and the AI probably won't be able to build it for you fast enough. the pyramid is a big help as well. Develop some core cities for settler factories, and some for rushing mounted warriors. Start a road in a few directions to get your units across the map fast enough, 6 workers can build one section of road every turn.
avoid auto-razing cities. attack after culture growth or population growth. be careful with the population as the AI will draft a unit (dropping pop), or they will build a settler. a size 1 city with no culture growth (must have at least 10 culture) will be razed even if it had more citizens before you started the war. Basically let your rivals do some of the work for you by letting them establish more cities. your horses can get there faster than settlers.
I had a few #1 spots using the Iroquois, until recently. I am currently working on a promising game that should reclaim my huge chieftan spot. (for good

)
a few other tricks i leave you the joy of discovering. look at winning games for some additional hints. BC victories already exist for Monarch and lower. A BC victory on emperor is well within reason, and i think ideally it might even be possible for a BC victory on Demi-god.