Assuming that you are not looking for any technical bike (very fast bike for road performance, compact bike for trail roads, expedition races or the eco-challenge) I believe a Dahon Curve or a Downtube Mini would be a really nice fit.

I am personally leaning towards 16" wheeled bikes instead of the most popular 20"wheel size. The ability to pack your bike for travelling using airport-legal luggage without the need of a degree in engineering or the patience of a monk is a big turn on to me, and having your bike with you more often is more important to me than being able to ride it on rough terrain. In other words, 16"wheels, for my kind of ride, is just perfect.

This is the downtube mini:



It has more gears than the Dahon, and most of the components can be upgraded from your local bike shop.

and this is the Dahon Curve (basic model, 3 speed, called D3):



It's cheaper, comes with fenders and rear rack, it has a more sophisticated folding mechanism and it comes with some devices to help keeping it folded once folded. This model only comes with 3 speed though.

For size idea, I'm 5'9", and this is my Curve D3:



Most people will point you towards the conventional 20" wheeled folders. The riding feeling of those are a little closer to what you are used to (if you ride 700c road bikes or conventional 26", "mountain bikes"), they behave better than 16" bikes on rough terrain and some brands (like Bike Friday and Dahon) make some super fast models that you can keep up with road bikers (assuming you could keep up with them on regular bikes as well).

Good luck on your decision. Please come back and let us know what you end up getting. The most important thing, in my opinion, is to have a chance to ride the bike you want before making the commitment to keep it. They are both good products and you can always return your Downtube if you are not satisfied with it (never heard of anyone actually doing it, it's a pretty sweet bike with several enthusiasts around this forum. I personally already had 3).