Deffensive fighting. I'd say Judo. If you can stay on your feet and throw attackers, you'll have a better chance with multiple opponents. Multiple trained opponents will still take you down, but for instance in the middle of a bar fight the ability to decisively remove opponents and remain on your feet is key.
Offensively, yeah Muay Thai.
One on one though, Jiu Jitsu is king.
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