Hey there,
This is the list of all Floors and Quests in order to help players to follow the "regular/supposed to be/but not mandatory" progression during a playthrough.

If you don't want to be spoiled,
Leave this thread NOW ! YES NOW !!
Well, it's up to you...

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N° - Section - Map Name - Quest involved (ending) & Boss :

01 - Cathedral - Floor 1 - None
02 - Cathedral - Floor 2 - The Butcher + entrance to Dark passage
03 - Cathedral - The Dark Passage (cave) - The Spoiled Water
04 - Cathedral - Floor 3 - The King Leoric
05 - Cathedral - Floor 4 - Ogden's sign*** + Gharbad the weak
---- END OF CATHEDRAL

06 - Catacombs - Floor 5 - The Magic Rock + Arkaine's Valor
07 - Catacombs - Floor 6 - Entrance of Chamber of Bone
08 - Catacombs - Old Cellar (Tristram / Cave) - Worm Infestation
09 - Catacombs - The Chamber of Bone - The Chamber of Bone
10 - Catacombs - Floor 7 - Halls of the blind
11 - Catacombs - Secret Tunnel (Tristram / cave) - Lost Treasure step 1 (the map)
12 - Catacombs - Soulstone Chamber (Unknown place) - Lost Treasure
13 - Catacombs - Floor 8 - Zhar the Mad + Horazon's sanctum entrance
14 - Catacombs - Horazon's Sanctum (Arcane Plan) - Became Arcane Palace in D2
15 - Catacombs - Horazon's Pit - Doomlock The lieutenant of Azmodan Back in D3
16 - Catacombs - The Protected Haven - Grimespike The lieutenant of Belial Back in D3 + The Summoner
---- END OF CATACOMBS

17 - Caves - Floor 9 - The black mushroom
18 - Caves - Floor 10 - Anvil of fury
19 - Caves - Floor 11 - None
20 - Caves - Maiden of Anguish (Tristram / Forest) - Andariel the Maiden Back in D2
21 - Caves - Floor 12 - None
22 - Caves - Island of the Sunless Sea (Netherworld) - Samael the Deathspeaker Corrupted Angel
---- END OF CAVES*

23 - Nest - Nest Floor 1 - Talk to Lester & Celia
24 - Nest - Nest Floor 2 - None
25 - Nest - Nest Floor 3 - Theodor
26 - Nest - Passage of Fire (cave) - None
27 - Nest - Valley of Despair (cave) - Izual the Fallen angel Back in D2
28 - Nest - Nest Floor 4 - Jersey's Jersay + The Defiler + Find the Crypt map.
---- END OF NEST**

29 - Crypt - Crypt Floor 1 - Grave Matters (upstairs at Tristram)
30 - Crypt - Crypt Floor 2 - The Dark Mage
31 - Crypt - Crypt Floor 3 - The Reconstructed note (found on floor 1, 2 & 3)
32 - Crypt - Crypt Floor 4 - Na Krul***
---- END OF CRYPT

33 - Special - Custom Secret Level - No, stop asking, there is no cow level !!
(Only triggered if you received the bovine plate from Lester's quest.)
---- END OF CUSTOM

34 - Hell - Floor 13 - Warlord of blood
35 - Hell - Floor 14 - Lachdanan (involve lvl 15)
36 - Hell - Pit of flames (cave) - None
37 - Hell - The city of the damned - The Cursed Blade (step 1)
38 - Hell - The Hellforge (cave) - The Cursed Blade (step 2) Back in D2 in Hell
39 - Hell - Floor 15 - Golden Elixir for Lachdanan + Lazaru's Staff + Lazarus Lair entrance + Floor 16 seal/entrance
40 - Hell - Lazarus Lair - Lazarus
41 - Hell - Floor 16 - Diablo
---- END OF HELL / GAME (back to Tristram you can export via the caravan exit.)

Notes :
*The Nest is supposed to be played at a similar level of Caves levels.
**The Crypt is supposed to be played at a similar level of Hells levels.
***Snotspill will be stronger if you gave him the banner.
***Na'Krul can be more powerfull if summoned in a bad way. Weaker if well summoned.