I go with SMF most times because it's free and works really well, or else Drupal because it comes with a ton of other features for a fully integrated website. Vanilla is pretty cool too!
I went with phpbb for now, later when I have some money to get a vB license I'll probably switch over. If you guys want to check out the forum and register and talk about sports and stuff, feel free to check it out at http://www.thedraftwatch.com
Bandwidth hungry? Just big is your site and how much data do you need to process? There are lots of features you can disabled in VBulletin to take some of the load off your server. As a vbulletin user, I do not understand your question?
That is because vbulletin is the best commercial grade forum softwre on the market. It has a lot of features (like paid memberships) that other forums will not include. As an example, some of the developers at mybb do not want paid memberships included with their forum software. For a commercial site, you want payment processing, moving from one group to another, reminders and demotions all built in - and all automatic. Go ask those phpbb developers if they have paid memberships (subscriptions) built into their forum. Can the PHPBB subscription manager change a members usergroups or grant access to a forum section once the payment is processed? Its kinda hard to sell paid memberships unless a lot of that is automatic, and built into the site. The modifications that you can get at VB.org area great. If a mod is found to have a security hole, it is pulled from the site. Plus, when you want support, if you have a current license, just open a trouble ticket. One of the benefits of using VBulletin, you get access to "real support" - unlike that free forum software where you have to post a question and hope that someone answers you. As for security, VB is maybe the most secure forum software on the market. Its not that VB is the best in one area, its the best in just about every area in forum software. I tried those free forums for over a year. Since I switched to VB, I look back and ask myself why I waited so long.
phpBB 3 is the best free forum software in my opinion. If you have the spare cash, then I would suggest vBulletin. Good Luck.
Vbulletin is by far the best, hook it up with vbseo.. and google will love your forum PHPBB 3 is not well protected against CFS nor is SMF.