Found in the following dictionaries
OTCWL2014 (NASPA Word List 2014)
"Winningest" Definition
adjective
Informal more successful or winning more often than any others of its kind: "it was one of america's winningest days for women” ( christian science monitor).
Source: American Heritage Dictionary
Superlative form of winning: most winning.
Source: Wiktionary
Most winning; most attractive.
Source: Wiktionary
Having the most wins.
Source: Wiktionary
Having achieved the most wins .
Source: Merriam-Webster
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