Stranded wire threads are tough. 

Strong, flexible and fine enough to fit through the holes in small beads they are a good choice for general stringing.  These threads are finished with French Crimps - for more information about using these threads see the Stranded Wire Thread free download.

Tigertail is an inexpensive generic stranded wire thread suitable for stringing beads 4mm-20mm in diameter and for making floating necklace designs.

Soft Flex and Soft Touch are wonderful premium brands with extra flexibilty and less tendency to kink.  You can pick from several diameters and colours.