I would always consider vBulletion over any other paid forum software's available online.

Yes it's true, that it is comparatively costly compared to the others but in terms of performance, administration, maintenance and user friendliness , it is outstanding.

Best part of vBulletin is that there are lots of plugins available which makes your work very very easy. You can easily tweak the board as per your wish. Again its XHTML compliant and you can also give the forum whatever look you want by means of skins and themes.