I would suggest the French, Germans, Russians or English.
Europe gives access to a relatively large amount of rivers, and most of them are on grassland. Rivers, especially on grassland, give a huge benefit, particularly in the early game. Getting 3 trade in a city allows you to allocate the last trade arrow to whatever you want (usually science), whereas having only 2 forces you to produce half taxes and half science. Building on a river can get you this for size 1 cities (only other ways are working trade specials or ocean, and the latter cuts other production too much).
Unfortunately, the European rivers don't help much for transportation (in contrast to the North American river system, which is a plus for the Sioux), but the grassland should make up for that.
The second advantage to Europe is that it is one of the best places on the world map from an early trade standpoint. Great Britain offers an offshore trading partner (if occupied), or a relatively easy place to colonize (and then send trade caravans from) if unoccupied. Also, Europeans can send triremes to North America with a much smaller detour than can any other civ, if you want a somewhat longer early trade route. Europe also doesn't lose any advantage in trade with America once caravels are developed, and should have easier access to the older (and usually better) cities on that continent (particularly if the Americans are there).
The French and Germans are roughly equal in terms of starting positions. They have river and access to more. The Russians have to walk a little more for the European Rivers, but they have a river of their own, and also will be able to expand in both directions, instead of just east.
As the English, all your core cities can get an offshore bonus with any city in the world when trading, so it saves you the trouble of building some good cities there in order to trade (though you do lose the chance of having someone else occupy England and have a close market for a lot of cities). You are also a couple turns closer to North America, if you want to send caravans or settlers that way, and your core cities are pretty well protected from aggressive neighbors (unless the Celts happen to be there, but you should then eliminate them post-haste).
The downside to England is that you only get 1 river square, and will have to build a trireme earlier than you might otherwise want in order to expand.
