If you are using it for commuting on public transport: Brompton. Nothing else folds as well and still rides ok. Get the two speed – good balance of lightness and having some gears, although I had to go for a smaller front chainring. Also get the bag that clips to the front, it actually seems to improve the handling.
A folding bike usually means you are getting more riding in, not less, which is a good thing. Plus, when the roads or trains all go to crap you can just hop on the bike and go. When I was commuting to London I was able to avoid massive delays quite a few times be riding an extra 10 miles. I’ve flown with mine on work trips and had mini adventures before a day of conferences and meetings.
Folding bikes are ace.