Best Atlas tree for more rare items

I want get more rare items, particularly divine. Which atlas tree is best for it?
More monsters - more chance drop rare items (Exarch)?
Einhar for more monster and build your own items or increase skill stone to +23% quality?
Blight for more items?

Item rarity does not effect Divine drops as it is currency. Item quant should. You are right that stacking up more monsters is a good way to get a higher chance of more Divines dropping.

Item rarity will help you get more rare and unique items dropping. Finding good items off natural ground drop loot may not be as easy as crafting them or trading for what you need. If you are still learning crafting mechanics trade might be a good choice. In my opinion most mechanics are pretty good relatively at least until you get up to a stage where they are getting very juiced up, beyond what I do myself. I don't have a good atlas to provide but it is certainly important to have something that synergizes, hopefully someone can provide something that is usefully for you.

If you are playing on your necro character I would say this league the mechanics that interact with the lantern are pretty good, most thing that add mobs to the map so if you get a good mod on the atlas you will get lots of drops off the pack. Also try and do league mechanics that you understand, some of them require a bit of knowledge to get the currency from. Expedition is pretty good for new players I think, if it fits well with your build, not being blocked by too many mods. I'm running ritual currently and I reacon at lease half the time I forget to grab the rewards before opening a new map, not so good for increasing Divines.

Exarch is fine, I've heard people say Eater is better but I think they are both good. Maven is also good for map drops but maybe less for juicing the map like your talking about.