If I had to pick then I'd say that this has been the single most useful thing I've ever printed. This dock firmly holds any normal sized smart phone and can mount to your bike handlebar, hang from your car rear-view mirror, or mount to a standard tripod.
In the photo I used it with a speedometer app to test my bike speed but my day to day use is for gps purposes.
Minimum 3 perimeters and 15% infill. Any material will work.
(1) hose clamp ~.5" thick (For clamping to a bike handlebar)
(1) hex bolt 4" long, fully threaded
(2) 1/4" hex nuts
(1) compression spring (any kind, just to keep the assembly tight)
(1) nylock 1/4" hex nut. (This one is inserted into the hook)