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Ronda Rousey is not the greatest female fighter on the planet, and never will be until she steps into the Octagon against Cris 'Cyborg' Venancio ... according to a former MMA champ.

Wanderlei Silva tells TMZ Sports ... everyone needs to calm down with calling Ronda #1, because much like Mayweather-Pacquiao -- Ronda-Cyborg is the only fight fans want to see.

Ronda's said she has no interest in fighting Cyborg ... due to the fact Cyborg was busted for steroid-use. But Silva -- a former PRIDE champion -- thinks Cyborg deserves a shot at the title. She hasn't lost since 2005.

Check out the clip -- Silva says Ronda doesn't need to fight Cyborg ... she HAS to fight her, and if she does, he'll "tip my hat."