The Russian Empire's expansion towards the east and south meant conquering lots of local tribes along the way (Buryats, Yakuts, etc.). These peoples unwillingly became ethnic minorities in Russia. Which group put up the best fight defending its territory from Russian incursion, in terms of the time or cost taken to subjugate it?
Apparently there is no consensus among scholars on this one, and several Pacific coast cultures substantially resisted Russian encroachment.
Beginning in 1640, the warlike Koriaks of Siberia's northeast allied with the neighboring Chukchi in the most successful attempt of any natives to oppose the Russians. [Lincoln, The Conquest of a Continent, pp. 54]
Stiffest of all was the resistance offered by the Amur River peoples. [Mote, Siberia: Worlds Apart, pp. 43]
... The Kolosh [Tlingits], like the Chukchi of Chukotka, were never fully subjugated by the Russians. [Gibson, Feeding the Russian Fur Trade, pp. 32]
Kazan was taken with severe warfare. Crimea also was taken with sever hardships. Then we also have North Caucasus.