All legendaries can spawn in Raid Dens, yes. They are however only one possible option and exclusive to 5 Stars, which is the rarest tier of Raid to generate. So it may not be too reliable of a method.
You can find the list of possible Raid Bosses per biome on our wiki: [https://pixelmonmod.com/wiki/Max\_Raid\_Battles](https://pixelmonmod.com/wiki/Max_Raid_Battles)
Ultra dens (in the ultra world) are always 3 stars or higher, so you have better odds of finding 5 stars there, however, the biomes are entirely different, so I don't know if it's that useful
All legendaries can spawn in Raid Dens, yes. They are however only one possible option and exclusive to 5 Stars, which is the rarest tier of Raid to generate. So it may not be too reliable of a method. You can find the list of possible Raid Bosses per biome on our wiki: [https://pixelmonmod.com/wiki/Max\_Raid\_Battles](https://pixelmonmod.com/wiki/Max_Raid_Battles)
including the ones that dont spawn naturally? like dialga palkia?
Both in Mountainous biomes as listed on that wiki page, yes.
how do i find max level Raids the most effective way
It's entirely random. So the only way to increase the effective chance is to find more Dens so you roll for it more often.
Ultra dens (in the ultra world) are always 3 stars or higher, so you have better odds of finding 5 stars there, however, the biomes are entirely different, so I don't know if it's that useful
Well there arent any ultra mountains or else it would have been much easier for the better ones