Donkey Kong Country: Tropical Freeze for me. Nearly every level has some great ideas, you don't get the feeling of repetition at all and the collectables are placed really good. Other examples that come to mind are Super Mario Galaxy and Metroid Prime.

Doom and Doom 2 had awesome level design too.

Mega Man, specifically Mega Man X, always had great level design.

Soulsborne. All of them.

Donkey Kong Country Tropical Freeze

I hope you don't mind if I pick specific levels. I'll try to stick to "design" as much as possible but I will probably end up praising levels that I liked without looking at the design XD

Super Mario Brothers: I always loved the tree-top levels with the bombs being shot at you from far away

Spyro the Dragon: Tree Tops!

All those supercharge ramps and all the different super tall trees you could fly too made for a super fun experience. High Caves was another great level with many supercharge ramps leading to otherwise-inaccessible caves. Misty Bog and Loft Castle were both damn good too.

Crash Bandicoot 2: The sewer levels and the ruins levels. I also really liked the levels where you rode the little polar bear <3 shout out to Kapi

Call of Duty 4: All Ghillied Up

Such an amazing level and experience. That moment when all the Soviet tanks and soldiers came walking right over you, and you had to lay still or else you were rekt; good times. You truly felt like you were in Chernobyl. The Soviet statues, the old church, the cars that you had to sneak under.... all in all damn good

Twisted Metal 2: Paris

You could blow up the Eiffel Tower and use it to drive around on the roof tops! Need I say more? The New York level was also pretty cool, and Tokyo with the subway tunnels and temple was also quite fun.

Uncharted: It's hard for me to pick out a specific instance, but if you ever wanted to find all of the treasure pieces in the Uncharted games you would end up having to do some pretty slick platforming. My favorite level design in Uncharted would probably have to be in Among Thieves where Nate has to solve the puzzle involving the giant gold knife-thingy.

Assassin's Creed 2/Brotherhood: Great level design. Trying to find handholds and other ways to reach the high ceiling of beautiful historical buildings was an amazing experience. 

Portal 2: Nobody can deny that Portal 2 had some of the most clever and brain-breaking level design in existence.

I'm trying to think of a horror game that used level design to its advantage but my brain is blank right now

Such a difficult question to answer. There are so many.

For giving the player numerous ways to approach a level:

Deus Ex, Deus Ex: Human Revolution, and Dishonored

For introducing and exploiting new gameplay mechanics as you progress:

Metal Gear Solid, virtually every Zelda game since A Link to the Past, Metroid Prime, Portal 2, Half-life 2

They should call the next souls game that xD

You should go back and finish it :P I just picked up the DLC which made me think of this thread, it's very very well designed.

Alundra