Who made all this?
The creator of this project is silec (silec#2387) but people all over the community helped by contributing information to finish this version of the project.

Do the Hell Bringer and the Fiery Spiderling actually give 0% or is this a typo?
Yes, they actually give 0% progression.

Are the monsters already grouped and just selected when you start a rift?
Yes, the monsters are preset in their monster sets. Each floor you will get 1 out of the 52 different monster sets.

Do champions, elites and minions give extra progression besides the progression orbs?
No, champions, elites and minions just award their normal monster progression + 3 (champions) or 4 (elites) progression orbs.

Does a single progression orb always award the same progression?
Yes, a single progression orb always gives you 1.15% (7.50 points).

Are there monsters that are tied to a specific map rather than being a part of a monster set?
Yes, those are the monsters that are tied to a specific map: Hell Bringer (Arreat Crater), Shock Tower (Pandemonium Fortress) and Bloated Corpse (Halls of Agony).
There are also monsters that are tied to a specific map but also exist in monster sets: Inferno Zombie (Halls of Agony), Blazing Swordwielder (The Keep Depths) and Spiderling (Caverns of Araneae).

Are there monster sets that are tied to a specific map?
Yes, those are the monster sets that are tied to a specific map: Monster set 4 (Icefall Caves), Monster set 49 (Icefall Caves) and Monster set 50 (Ruins of Corvus).

Could you add information on the topic Progress vs. HP?
Yes, there is a spreadsheet for those that are interested in this data. Click here to view the spreadsheet. Keep in mind that every monster has the same base HP and the table shows their multipliers. For the column "Progress / Average HP ratio" keep in mind that this calculation only factors in the progression and the average HP multiplier of the monster (this means that factors like monster mechanics, monster damage and hitbox size are not included).

Which of the monster sets are considered to be the best and why?
There are multiple monster sets that are really good. Click here to view a tier list created by wudijo and silec but keep in mind that density of certain high progression mobs, density in general, monster damage, monster mechanics, hitbox size and the map layout are huge factors to determine whether or not you are fighting a good monster set. It is also different if you play solo or in a group.