Freestyle aka Front Crawl. ...but what may be fastest generally not be the fastest for you personally. I haven't timed myself but I think my backstroke is perhaps my fastest stroke. At least, it feels that way. I don't aim for speed though...I'm not swimming as a competition, just for fun and health. I have friends who prefer other types of strokes, i.e., breast stroke. So it depends on why you are swimming and what your goals are.
General conscencous is freestyle - because there should always be continuous propulsion from your arms. That said depending on your speciality other strokes may be faster, also it depends on the distance being swum.
For 100M I think the typical order is: Front crawl; Butterfly; Back crawl; Breaststroke
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