Hi, I was wondering if anyone has any experience with an external level editor, if so which one and how would you implement the level into a game?
I am currently building my own for a 2D time-travelling platformer, based on Windows Forms. Everything is coded manually, it is a lot more work but a lot more control too when it comes to use it in your own game. 
But it is not that much work, i almost finished it and i have been working on it for 2 months now… but this is because i’m in highschool (17 years old hehe) and 90% of my time are for school… So it wouldn’t be that hard to make your own if that’s not a problem (and if you have experience, of course).
What kind of editor are you willing to have ? (A 3D editor, 2D platformer…)
While I have never used 3rd party editors for my own games, I have made and used my own editors…
…And my experience making levels for other games, with their appropriate tools, tells me that can be a feat in itself… Not so much a short-cut…
My thinking is, that whatever editor you use, you have to create a reader-class for the file anyway, so you have to learn everything the editor outputs… And then figure out a way to apply the data to your various classes…
Thats going to take dedication as well…
One thing I did was create a game that reads levels from text files… (you could find a better format)
-That way you can design whatever editor you want around the requirements of your game, rather than the other way around… -And just have it spit out text files… This way you can also tweak the files by hand, or inspect for bugs…
This worked very well…
Also, you can leave your editor fairly basic/general, and achieve variation through scripting…
Thus while using your editor for tedious things like coordinate lists, you can do more one-off things
things manually through code… Or scripts…
This keeps your editor small and managable… And you can expand your game, without having to update your editor as well… … …
Example: My editor for a breakout game exports brickID, geometry, location and other basic data…
To do various advanced things, like having moving bricks, or lighting sequences, I use a hand-written effect.txt files that accompany each level file…
I definately recommend not relying ONLY on whatever editor for your level-data… I found it SO usefull to be able to combine sources when loading a level… It changed everything, and made updating much easier 
If you are after an editor that is quick and easy to use and exports into a easily understandable format (XML for instance), then I would recommend Tiled. It’s quick and easy to get set up with and there’s plenty of importers for the output that it generates out there.
Otherwise, I think there’s a few people in this community who have spent time integrating MonoGame into WPF controls and creating an editor with a combination of the two.