Who made all of this?
The creator of this project is silec (silec#2387). The programming was done by Basti.
Special thanks to all former warpath members, the members of Zero Empathy, d07RiV, Quin69 and everyone else I forgot to mention that contributed to the project in any way.

Explanation: Progress in Points
Internally Blizzard uses a point system to specify monster progression. Dividing the monster's point value by 650 (100% equivalent to reach the Rift Guardian) and multiplying it by 100 will give you the progression in percent.

Explanation: Average HP Multiplier
Monsters always have an HP range and with the following formulas we can calculate their average HP multiplier:

Max HP Multiplier = Min HP Multiplier + HP Delta

Average HP Multiplier = (Min HP Multiplier + Max HP Multiplier) / 2

Explanation: Efficiency
The higher this value, the more efficient it is to kill the monster. Keep in mind this calculation is purely based on monster progression vs. average HP (Progress in Points divided by Average HP Multiplier), ignoring other important factors.

I encountered some monsters in Greater Rifts that aren't included in your tables, why is that?
The tables do not include Elites, Champions, Minions or monsters that are spawned by them. This is because Elites/Champions aren't killed for their raw progression but rather for the progression orbs that drop after their death. Minions are usually only killed through collateral damage since they have a high health pool and only award raw progression. Elites, Champions and Minions also appear in a large variety of monsters that don't exist in Greater Rifts otherwise, which would clutter the tables unnecessarily, showing you monsters that you might not even encounter once after getting a Monster Set multiple times.

Besides that, the tables have a complete collection of all monsters that appear in Greater Rifts and their related Monster Sets, including monsters that are spawned by other monsters after their deaths (e.g. Grotesque > Corpse Worm), while they are alive (e.g. Foul Conjurer > Accursed Hellion) and monsters that only appear in certain maps (e.g. Shock Tower in Pandemonium Fortress).

Does the Hell Bringer actually give 0% progression or is this a typo?
Yes, it actually awards 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 33 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. These 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 is also the Spiderling (Caverns of Araneae) that is tied to a specific map but also exists in Monster Sets.

Are there Monster Sets that are tied to a specific map?
No. There used to be Monster Sets that were tied to specific maps but they either got removed from the game or were made no longer map exclusive.

Which of the Monster Sets are considered to be the best and why?
There are multiple Monster Sets that are really good. Density of certain high progression mobs, density in general, monster damage, monster mechanics, hitbox sizes and the map layout are huge factors to determine whether or not you are fighting a good Monster Set. The build of the character that is being used is another factor that plays a big role. It is also different for solo and group play.