which is the best speakers for marantz amplifier 6012

[RRR]

That's because the same group owns both including Boston accoustics.
There are many other brands that go well with Marantz like Quad, Wharfedale, Dali, Monitor Audio, Kefs etc.
Hearing them for yourself if possible with different brands is the best option

So true. Earlier only Rotel used to be suggested for their speakers.