There's no simple answer to your question, as it depends on your requirements.

- What's your budget?
- How big do you think the forum will become?
- What features you you require?
- What level of technical knowledge do you have?
- Which server-side languages do you know?
- Do you need the ability to modify the templates or merge them with an existing layout?
- Do you require a unique sign-on membership with your current site?

I used to use PHPBB but switched to vbulletin. If you go with vbulletin it is not free (160.00us at time of this post) but it is WELL worth it.

After I got aquainted with it I converted my other boards to it.

*If* you go this route, get VBSEO plugin. It works greats! costs about the same as vbulletin though ;(

Best bulletin software on the market imo and I have tried many of them over the years. ;)

Thank you MThiessen and Encylo,

Yep, I should provide you some of our requirements:

The price is not our main concern.

The forum may run on Linux
The forum's skin is easy to change, modify.

But to MThiessen's experience, We may now think of Vbulletin.

thank you very much.