A flexible filament that is much easier to print than most others.
PolyFlex is a really nice all round flexible filament that makes for great functional flexible parts like tyres and grippers. It's not quite as rubbery, soft or grippy as something like NinjaFlex, but it makes up for this by being a good deal less prone to buckling in the extruder and generally is a very easy to print material, in fact it is much more like printing "normal" filaments. Decent speeds and flow rates can be achieved without the notorious buckling of other flexibles.
PolyFlex can be dual extruded alongside rigid materials such as PLA to create objects with selective compliance.
PolyFlex requires no special print surface, does not need a heated bed to print without warp, and prints at fairly easy temperatures of around 230C.