Game with the best level editor? Or the worst?

[CreamyHerman’s]

Farcry 2 had a pretty good level editor

I see your Far Cry 2 & raise you Far Cry 4

 

[XYZpdq]

Lode Runner

 

[loli]

Thief 2: The Metal Age came with a nice editor and has a community that still makes stuff to this day I think. It's one of the best games of all time.

 

[Blood Bath & Beyond]

Neverwinter Nights 1, Neverwinter Nights 2, Thief 2 and The Dark Mod if you are talking about games that come with their own editors, but there are a lot of games out there with huge modding communities that have made their own custom tools that can be just as good and often better than the stuff provided by actual devs.