Which locations have the most and hardest enemies in Skyrim to assist levelling up?

Solution 1:

There not many enemies that are more tanky than Draugr Deathlords/Overlords and Forelhost is one of the larger of the Nordic Ruins. If your goal is to level up skills, you mostly just want to face enemies with lots of health as skill advancement is based on damage dealt so sticking with Draugr is probably a good idea as they have more health than some of the other typical enemies you find in the game.

If you want to fight hard and annoying enemies, the Dawnguard DLC adds the Falmer Warmonger to most areas where Falmer can spawn. They are very tanky and hit hard.

You can try searching through the various lists of ruin types on UESP for more ideas:

• Nordic Ruins (Lost Valley Redoubt is a great area filled with Forsworn)
• Dwarven Ruins (Blackreach is an extremely large zone, as is Mzulft)
• Caves (The DLCs adds a lot of largish caves, the Dawnguard DLCs adds Falmer infested areas in large numbers, like the Darkfall cave/passage)

You can also go Dragon Priest hunting as they're typically in medium/large burrows with large amount of Draugr.

Solution 2:

There was a poll done on the hardest locations here:

https://gamefaqs.gamespot.com/boards/615803-the-elder-scrolls-v-skyrim/68628790

The poll did not yield many votes, but, with some research on UESP and reading round, these are generally considered the hardest. I have tested them with regard to how effective they were to levelling up:

• Forelhost [Nordic dragon priest lair, min level 24: a whole load of Draugr Deathlords and Overlords, esp at the end you will have to fight four Draugr Deathlords simultaneously and the dragon priest Rahgot. Tested - very effective {+2 levels}]
• Volskygge [Nordic dragon priest lair, min level 24: as above, but fewer. Tested - effective {+1 level}]
• Darkfall Cave / Forgotten Vale - [Falmer cave / region: a load of falmer of all levels + frost giants [the latter do not respawn]. Tested - effective {+1 level}]
• Tolvald's Cave [Falmer cave, min level 16: falmer of all levels. Tested - effective {+1 level}]

• Sightless Pit [Dwemer, min level 18] [tested - not effective]
• Lost Valley Redoubt [Nordic ruin / Forsworn] [tested - not effective]
• Morvunskar [Mage fort, min level 8] [tested - not effective]
• Ansilvund [Nordic, min level 8] [tested - not effective]
• Arcwind Point [Nordic, min level 6] [tested - not effective]

All of these respawn within 0-30 in-game days. One can therefore rotate between the effective locations using them to level up.

Of course, it also depends on how ones plays these areas - levelling up 2+ skills at a time [by making one or more legendary if necessary] is obviously going to yield better results.

Labyrithian and Bthardamz are often mentioned, but these locations do not respawn. Referring to the answer given by the user l I, I have looked at [and visited] many of the medium-large caves/dwemer ruins/tombs and found them to be largely ineffective - even though they may have multiple enemies, these locations were often level-locked so made little difference in late game. Also most of the dragon priest tombs do not respawn [there are three that do, in addition to the two given above there is High Gate Ruins, but this is not a difficult tomb to clear].

Furthermore, if one mods, or even going vanilla, one may consider the acclaimed "High Level Enemies" mod on Nexus as a sole addition, which adds more scaled enemies for all types and should be enough to keep things interesting.

EDIT: In the end I was on the right track with the tombs but doing things wrongly - one does not needs hordes of draugr to level up a warrior-like character, one single draugr is enough if you play a defensive game with it. I came across a Draugr Death Overlord in Arcwind Point yesterday and, through using a combination of block, light armour and restoration [having made the first two legendary], managed to level from 78-80 in about 15 mins. I will select this as the right answer since I solved my own issue, though user4951 still keeps the bounty since his answer was interesting and gave me lots to think about [and also the bounty can't be changed, but he would have kept it regardless since I appreciate his effort].