It depends on your current troop research and playstyle. The best are Germans and British. You should choose if:
British:You only need to upgrade ranged infantry and artillery when advancing ages, leaving the oil for the Airplanes and Generals.
Germans: I think they're best at Atomic Age, but you'll have a ton of researches that require a lot of oil (4 bazooka upgrades, APC upgrade at library, university upgrades require oil), etc
So if you want the easy way to the top attack, go with British. If you're willing to farm millions of oil to have the strongest attack, Germans.