The coders. Best in the industry, IMO.
Assuming you're including 3rd party developers in that; Only because the XF devs recognise the value of 3rd party developers and have made XF2 an absolute dream to develop for
Unlike certain other forum platforms where issues preventing 3rd party devs from extending 60% of the API remain un-fixed in spite of being reported during the developer beta period 6 years ago, XF2 has been designed from the ground up to be extendable.
There was one single case I've personally come across where something couldn't be extended (Sessions) and that was changed at some point during the developer beta after another coder reported it and I chimed in in support of getting it changed. Two people is all it took for the developers to apply the change needed, even though I'm sure they had more show-stopping issues to address at the time. Paraphrasing what the devs said at the time, they simply hadn't realised that there was value in extending the session class in that way, but our use cases were valid so we got the change we needed.
I've recently completed development of one of the biggest projects I've ever done - approx. 2.2 MB of pure text (not counting images, templates, phrases, or JS), that's approximately 1
billion characters of code - and as far as I can remember I've not come across a single instance of wishing it was easier to do something. On the flipside, I have come across a whole bunch of instances of "I can't believe it was this easy to do that".
The biggest example is probably the search engine; unlike other forum software where in order to sort out the integration you need a ritual dagger, a pentagram drawn in your own blood, as well as a sacrificial goat, in XF2 you simply complete 4 form fields in the AdminCP then add a file to the file system. How big is this file? 7 KB.
To put that into perspective, the icon file I use for all of DBTech's mods in the XF2 addon list in the ACP weighs in at 12 KB.
Completing integration with what you would think is the most advanced piece of functionality on a website requires you to write less information than is contained in a 96x96 PNG image.
If that doesn't blow your mind then I don't know what to tell you.
Fillip