A very popular combination is the Brompton folder with speed-drive. It needs a +6.5 mm cone ring to allow folding (the rear chainstay folds where normally the gear housing is located) and a wider nut on the left side to make up for the additional width.
A good set-up is a speed-drive with a 34 t. chainring and trouser guard. With such a chainring, you have much lower gears for climbing (34 instead of 50 t.) but also much higher gears for speed (56 instead of 50 t.). Any other combination is up to your personal preference...