The most classic in my opinion is the black City. It does depend on what size you want, though. The First was the first, lol. It is much smaller than the City, so it does depend on what you want to carry. Trying on the different sizes is the ideal to see what you will like as all Bals are gorgeous and "classic" in their own right
ETA: If that is not possible (it isn't with me as I have no stores in my area that carry Bal), the "pictures wearing your Bals" thread is very helpful.
I agree with the others, the Black City with regular hardware is THE classic. However, if you don't carry much with you regularly, you may want to consider the First. One other consideration is the Town, it's size is right between the First and City, it has a shoulder strap that you can wear messenger style or double it for shoulder use. Hope this helps.
Between the First and the City, I'd pick City as a first bag, simply for its bigger capacity.
If you like how the First looks, but want more capacity plus the advantage of a cross-body strap, then how about the Town? I bought one, initially thinking it would be an occasional-use bag for me. Surprisingly, considering how much I normally carry, I've found the Town fabulous! I only need to carry my sweater or leather jacket on my arm, as opposed to when I carry my City, which can fit either in.
Being the size in between the City and the First, the Town is definitely worth considering and trying out if you don't carry too much around (or are a clever packer) and would like a cross-body carrying option.
another vote for black city, RH!!! its the most famous and sought for color, including hardware...
now, having said that i sold my black city as i found it too small for everyday.. i mostly have work bags and one part time, one first for going out at night.
so, if you want to be more specific, you can tell us more about what you would need this one and only one balenciaga bag for...
what are your other black bags? small , medium or large?
although you can't go wrong with black b bags... there is another colorful world to balenciaga that you might want to hear about!
I also vote for the black city as well, it's a classic. I also like the size of the city better than the first because it's bigger, but that's JMO. If you don't carry a lot of stuff than a first might work for you. When I started looking at Bbags I originally wanted a first but after I saw it IRL I knew it would never work for me it's way too small for my needs and I don't really carry a whole lot.
Although I am not a fan of Nicole Richie (not because she isn't cute - just because I have never even heard her speak, lol), I love this picture of her and her well-loved Balenciaga Black City. It is just so carefree yet stylish. You should, of course, get the bag that speaks to you!