A folding bike is a compromise between ride quality and foldability. Moulton makes great artisan folding bikes with very unique design. Brompton also makes lovely folding bikes (previously-reviewed), but kind of artisan and pricey. I like the previously-reviewed Strida if all you have to do is ride 1-2 miles to the transit station. It’s not much good if you have to ride for more than 15 minutes. Citizen Bikes are awful, but some people who have never ridden a nice bike seem to be able to tolerate it. Dahon is starting to make some pretty damn good folding bikes at reasonable prices.
But my favorite is Bike Friday. It can fold into a suitcase that won’t incur over-charges on airplanes. Super light. Rides like a real bike, in some ways better. They have a few different models (even tandems!); I’ve ridden most of them — they are all good. I optimized my choice for quality of ride, but you can build them with ease of folding in mind by specifying what you want in terms of tools/no tools. For instance, some models require tools to fold for airline travel, but not for folding to stash on cars/buses. The Tikit models, on the other hand, explicitly requires no tools for folding at all.
These bikes are not cheap. I am a self-admitting bike snob. I value ride quality. Most low-cost folding bikes just feel cheap. The difference is in the custom-fitted frame, and better design details, higher-quality components and etc. (Bike Friday has been doing it for years). But you can get on a good Bike Friday for $1200. If you want, you can spend up to $3000 or even more for extras, but the frame is the same. These guys have great customer service, too.
I love mine. When it was recently stolen, I was heartbroken. Bike theft is like pet death. If you see my yellow Bike Friday (it has my wife’s name “Arwen Griffith” on the top tube), throw rotten fruit and stones at the asshole who stole it.