Best forum solution for a growing WP site?

Hello.

Im working on a site that needs a forum. And i have looked around and read some about some solutions, but so far nothing seems to be the thing that sticks out. If the project goes as planned it will be a forum with maybe not so many users in the beginning but within a few months its exspected to grow quite big.
I have tested buddypress with its “group forum” system, and it looks quite boring. Iv tested Mingle-forum and it looks much better, but im not sure if it has even close as many functions a normal phpbb forum has.

So my question is simple, what would be the smartest solution to go with to integrate a forum to the word-press website? It needs to be included in the sites design and not be its own site with a simple link to it from the wp-site. It should be fairly scalable, and also not to heavy on the servers.

How have you guys done this?
All input, information and pointers are most appreciated.

Best regards
Peter