vb or ipb are the best, both are payed though.
Both vb and ipb are very secure, lots of skins are available free and paid and there are also 10000's of free modifications that you can install in seconds.
There is one free option i can recommend and thats is "smf" simple machines forum.
Although a fair bit like phpbb, its alot more secure
phpbb on the other hand has alot of exploits, not sure about the latest phpbb3 but anything phpbb i stay clear from.