In Path of Exile, your final task is to complete endgame grings. The following guide are somewhat difficult for people who only want to spend a short time, because this is for players who willing to spend 2 months + and aim for 40/40 challenges, if 36/40 just ignore this. For players who wanna do this.

Atziri, Queen of the Vaal are worth doing them it gives profit. Uber Labyrinth brings a lot of profit but I know some players got tired of passing the traps over and over again.

Shaper or any of the Shaper's Guardians always worth doing them as they give profits, but 150 sound too much, maybe people who like to farm it won't have problem for this.

Elder or any of the Elder's Guardians this is part of new core mechanic of expansion. They spawn quite often when there's enough elder's influence in your atlas. There always 4 type of guardians spawn together with The Elder in the influence area, which also means that you only need to beat 25 times of influence phases.

There's also another trick to do faster which is having 2 character method, 1st character open the map run to the boss then switch to 2nd character kill the guardian, then open map again, switch, kill, repeat, and ya the guardian won't disappear by doing this method.

Bestiary Bosses 20 sound do-able, could try make a trustable RAID(party rotation) to do this easier.

I would pick Uber Labyrinth cuz I like them not to mention they buffed the loots even more in 3.1, Atziri cuz they're simple(for me) and cheap, Beastiary Bosses cuz is 20 only not that much, and Elder guardians.

The reason I choose elder over shaper is because there's a tricks to make his guardian always at there won't disappear unless they fixed it already, if so then, we got no choice but to farm shaper, buy many T16 maps then farm them.

There's a lot more players guide on U4GM, so be sure to check out if you need some guides. Keeping an eye on poe buy chaos, you will get more gains.