I need to create a support forum for my clients, where all questions/answers will be organized and searchable. So, obviously I need a forum. I am looking for a solution that will allow users to create a thread with their question, and in a separate text field they can enter private details like their wp-admin / Cpanel details that ONLY admin can see. Anyone know of anything like this off hand? I am willing to pay, of course.
Way back when I had clients with forums I did something like that with vBulletin. You should be able to do that with any decent forum system.
XenForo and VBulleting are great. One of the main advantages over others is related to SEO. For example phpBB has very unfriendly URLs for search engines, which implies lower traffic from them.
As R. Martin said, XenForo and VBulletin are your best bets. Great communities behind each product and lots of mods, themes and other goodies, not to mention the friendly URLs. Both do cost you some coin, but in the long run, these can be mitigated through your sales, donations or however you run it. I had used VB 3.x for many years up until php 5.2 (i believe) became the new standard forcing me to upgrade to 4.x. If you carry a subscription you won't need to worry about these lapse's in keeping up with technology. For a freebie forum, SMF however their website could use a face lift.
I haven't explored xenforo all the way, but I think I saw an option (i have xenforo) that allows me to put each thread independent from each member. Meaning that each thread could be private, independent from other members. I guess that could work as a helpdesk for your clients. But I would recommend to use a helpdesk software rather a forum script for this type of client service. The main purpose of any forum is the multi-user communication. A helpdesk software that integrate with you forum is the way to go. You could also request to do a custom modification, but that would cost you more than getting a really good helpdesk script.
Personally, I think that XenForo is the best forum software out there. It is mobile friendly and it is snappy and fast.