Some questions to ask yourself to help you decide:
1) How much do you want to spend? How much are you willing to spend?
2) How small do you need the bike to fold? Almost all websites for these bikes will tell you the folded dimensions of the bike and show a picture of it folded.
3) How important is weight? Do you need it to be under 25 lbs., under 23? Would 30 lbs. be alright? If you're carrying it a lot, this will be important, but, other things being equal, low weight implies a high price penalty.
4) How long will you ride the bike at any one time? Many small wheel bikes are fine for short distances but get very tiring and uncomfortable over long distances. And how important is comfort for you? Generally (but not always), bigger tires, longer wheelbases, and suspended frames/seatposts/forks will make for a more comfortable ride.
5) Will you ride on rough terrain (not sure what kind of park trails you mean)? If the answer is yes, this will probably cut down on the likely candidates. And how wide a range of gears do you need? If all the terrain you're on is always flat you might get by with a one or three speed.
6) How difficult are you to fit? One of the best things about folders is the wide range of sizes they can be adjusted to, but some are better than others for the extremes.
The problem with folders is that they are hard to test drive. No shop near me has any sort of selection. Before buying mine I rode a couple that clearly weren't right for me. I ended up ordering one without riding beforehand, based on reading the posts in this forum and how people described its strengths and weaknesses (as well as the price I could get it for). It turned out to be an excellent choice for what I wanted, but would be wrong for others with different priorities. Read a lot of the old posts and you'll get a pretty good sense of what each bike is best and worst at.