That's my list:
1) XenTag is one of the popular addons of Xenforo, and it's free. This integration is IMHO very important because without that it will be very confusing to have tags that haven't nothing to do with the forum niche...Probably we will have the possibility of category
2) Who Has Visited in Last 24 Hours very important, useful and popular in Xenforo, it makes visible the members that has visited your forum in last 24 hours. This should work in couple with the Users Online per Forum feature...i need to see for every forum *ONLY* the users online and the users that in last 24 hours have visite this particular forum...i have one big forum and 3 little forums so it would be useless to share the global data among them. VERY VERY IMPORTANT IMHO to have this feature and addon integrated in Multisite.
3) Credits Premium: Very important to have an unique currency for every forum.
4) Store: good addon in conjunction of Credits addon by Brivium. Simply the integration here is the possibility to have one category (and subcategories) for forum.
5) Social Groups the most popular (and free) social groups addon. Should be possible to have different category for every forum...
That's are the most important addon integrations for me...what's yours? ;-)