Woltlab Burning Board is good
woltlab.com
Is there a nulled version for it and is nulled easy to hack??
Well of course many will vote for vbulletin as it is very famous and hence even if there is a security flaw in the server itself then people will think that its due to vbulletin. Rule of thumb, secure your server and keep your vbulletin updated and you are good to go.