Skyrim:100% Completion

There is no message or alert that lets you know that you have completed everything the game has to offer. So, here is a list of all completable statistics within Skyrim; it is up to you whether or not you want your character(s) to achieve these ultimate measures of completion.

Quests
Technically, there are an infinite amount of quests in the game. This is because several Radiant Quests can be completed an unlimited number of times. For the sake of this guide, we will be including unique quests that can be completed only once, or a series of radiant quests that ultimately can be completed because they are not infinitely repeatable.

In order to complete every quest in the main questline, the Civil War questline must not be completed until after the completion of the main questline, otherwise Season Unending will not be available to start.

Each player can only ever be on one side of the Civil War during that questline. By joining either the Imperial Legion or the Stormcloaks, you will forfeit the ability to join the other faction. You can only change your mind once at the ending of the The Jagged Crown by giving the Jagged Crown to the lead figure of the opposing faction: give the crown to General Tullius if you want to be in the Imperial Legion or to Ulfric Stormcloak if you are wanting to be a Stormcloak. 

Several quests during both sides of the Civil War questline may be replaced or unavailable altogether depending on the outcome of Season Unending in the main questline. It is suggested to complete the Civil War questline as late as possible in a playthrough as it may lock out several quests, namely favor quests for Jarls which will be replaced once their Hold has been taken over by the opposing force.

In order to complete every quest in the Dark Brotherhood questline, Destroy the Dark Brotherhood! must not be completed. Additionally, Delayed Burial must be completed before Innocence Lost otherwise it will not be available to start. Finally, in order for Locate the Assassin of Old to start, the bonus objectives of Breaching Security must be completed. See Quest Conflicts for more examples.

Main - 20 Quests
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#*  (only available if the Civil War questline has not been completed)
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Civil War (Imperial Legion) - 15 Quests
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#*(umbrella quest)  
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#  (will not be received if Riften is handed over to the Imperials during Season Unending)
#  (will not be received if Winterhold is handed over to the Imperials during Season Unending)
#  (will only be received if Hjaalmarch is handed over to the Stormcloaks during Season Unending)
#  (will only be received if The Reach is handed over to the Stormcloaks during Season Unending)
#  (will only be received if Falkreath is handed over to the Stormcloaks during Season Unending and if you initially side with the Stormcloaks but turn in the Jagged Crown to the Imperial Legion)
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Civil War (Stormcloaks) - 14 Quests
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#  (will only be received if Dawnstar is handed over to the Imperials during Season Unending)
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College of Winterhold - 18 Quests
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Companions - 8 Quests
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Daedric - 15 Quests
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Dark Brotherhood - 29 Quests
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#*(side quest)   (only available if the bonus objectives of Breaching Security have been completed)
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Thieves Guild - 20 Quests
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Dawnguard - 15/16 Quests
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#*Join the Dawnguard.
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#*Join the Vampires.
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#(umbrella quest)  
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#*(side quest)   (Vampire side only)

Dragonborn - 7 Quests
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Other than the infinite radiant quests, there are a total of 273 quests of all types.

Collectibles
There are many different things to collect or obtain, whether it be unique armor, to learning all the spell tomes, you will spend hundreds of hours just trying to collect everything.

Stones of Barenziah - 24 of these gems are located throughout Skyrim, and you have to find all of them to restore the Crown Of Barenziah to its former glory.

Bugs in a Jar - 5 jars with live bugs inside are scattered throughout Skyrim. Each jar also has a unique inscription etched into the inside of the lid.

Dragon Priest Masks - 9 of these masks are found in the province of Skyrim, and 8 of them are obtained from the Dragon Priests themselves. One more mask is required to obtain the ninth which is available when the first 8 are brought to the Bromjunaar Sanctuary and placed on the shrine there. This makes a total of 10 masks, although only 9 will ever be helpful in combat since the tenth is only used to acquire the aforementioned 9 masks. Four more can be acquired from the Dragonborn add-on; although you will encounter 5 dragon priests, one of them will not surrender a mask.

Dragon Claws - 10 of these claws are in Nordic Ruins, and each is unique, being made with different gems or materials. One more can be acquired from the Dragonborn add-on, although the claw is actually split into two parts.

Shouts/Words of Power - 20 Shouts can be used by you, and every shout has 3 words of power, so there are 60 total words of power you may learn. Four more can be found from the Dragonborn add-on, making a grand total of 24 Shouts, and thus, 72 words to be learnt.

Daedric Artifacts - With 16 out of 17 Daedric Princes being in Skyrim, you can obtain 16 Daedric Artifacts from their respective prince.

Every Armor And Weapon Set - There are many different materials, and they are used to create different sorts of items for your travels.

Every Spell Unlocked + Mastered - There are 5 schools of magic, and every one has many spells. Alteration has 17 Spells, but 15 without add-ons. Conjuration has 33 spells, but 25 in vanilla. Destruction has 28/29 (depending if you are a Vampire).