Megaman, specially X. They're not hard levels but the placing of items, monsters, power ups and is perfect imo. The first stage is beautifully coordinated so you learn all basics while playing and without tooltips showing up.

Super mario Galaxy's level design is as good as it gets.

Can't really pick one. It's be easier if you asked "on X platform" or "X genre"
Candidates would be Deus Ex, Deus Ex Human Revolution, ICO, Quake (and about a few other old school shooters), Resident Evil 6, MGS2, DmC, Klonoa 2, and others

As far as Bloodborne goes I think Old Hunters DLC did it better than the main game

The Warcraft 3 campaign.

SM3DW levels are almost flawless imo regarding platforming. But the most impressive level design was Dark Souls for me. I was amazed everytime I opened a door and said: "Look where I am! :O".

Every game from From Software has amazing level designs!
From Software makes the best level designs imo.

If I had to choose one game it would definitely be Dark Souls.

Isn't the game with the 'best level-design' usually also one's best/favorite game period? I mean, it should be, great level-design should be a pretty giant piece of the favorite-pie..

There wasn't really anything to unlock aside from one single small room with an item on it when you have to travel all the way back for a PSG-1. There's really not much (i.e: just a new room) being unlocked when the game once again in the same spot teleport Snake back to the very first place for an albeit cool torture scene. Finally, you get to overcome that first Sniper Wolf part and you're greeted with an (almost) never-ending upper-staircase with spawning enemies. Cool rappel scene afterwards, then you get to a boring building that gives you a missile launcher out of nowhere and once again you have to climb another set of never-ending stairs, this time with turrets. And then go down!

And let's just not talk about getting the PAL Key, using it on the computer, backtracking to the frozen stage, going back to the computer, using it again, backtracking to the lava stage, then finally going back to the computer. Seriously, no, the design of MGS1 isn't really good.

Super Mario Galaxy 1/2, Resident Evil 4, Super Mario Bros 3.

Demon Souls had really good level design, you could feel they really put a lot of thought and craft into the world. It was clever how they all linked together aswell, I say the same for Dark Souls as well.