Depends on your team tbh. The battle institute gives 16 BP max for 5 battles, only the last 1 or 2 presenting any challenge whatsoever. One runthrough of singles at the Battle Maison gets you something like 48 BP (IIRC) for 20 battles, with only one or two if the last five being tricky. Super Singles/Dubs at the Maison gives you probably the most BP - 2 per battle + 1 per streak of ten. So 20 BP for the first 10 battles, 30 for battles #11-20, 40 for #21-30, etc. If you've got an OP team like Speed Boost Blaziken, and other Uber-worthy(butnottechnicallybanned) pokemon, go Battle Maison.
To add a small thing to that, the Truant Durant Entrainment strategy alongside a Quiver/Dragon Dance + Protect partner also makes it easier to get a long streak going and get easy BP. Really time-consuming though, so it's only really worth it for later rounds or once you reached the 7 BP per win cap. But yeah, I second this post.