I would say the Pacific Carryme requires most maintenance (Looking towards New York and Germany before running for cover!)

On a serious note, I have had no maintenance issues with my Brompton and found it very reliable. After 10 months extensive use I would say the most it needs is a good cleaning of the gears and regreasing of the chain.

The Downtube 8H I own developed a hairline crack at the seatpost but that would happen on many alu frame folders as I had it slightly above the minimum insertion point. Of course I did find the seatpost wasn't really long enough and I'm 6 feet tall exactly. My Downtube FS proved to be very reliable and a taller frame but I did have problems with the bearings in the folding pedals developing clicking noises within a few weeks of use and the left crank fell off cycling. Once I addressed these issues it was more or less a trouble free bike.