We don't really keep track of the size of forums, let alone "largest" forums (the UBBDev team in charge of updates isn't staff of UBBCentral, so we don't really keep track of anything).

Some decent sized UBB.threads forums offhand would be
http://backofthenet.com
http://koptalk.com
http://divorcebusting.com

A better listing would display if you were to search a major search engine, like Google, for "Powered by UBB.threads™ PHP Forum Software" (the notice at the bottom of each forum installation which isn't paying for debranding).

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