What is the hardest slitherlink board you have seen/solved/tried to solve? I've tried googling for some hard ones but came up with simple ones or sites describing what slitherlink even is... So in other words - please provide examples of some hard/hardest boards you know (standard slitherlink rules but it can consist of any shapes - doesn't have to be squares).
This website has the hardest slitherlinks that I know of: http://kwontomloop.com/puzzle.php
Also, as mentioned in the comments by Eric, https://www.gmpuzzles.com/blog/category/loop/slitherlink/
has really good slitherlinks
http://www.janko.at/Raetsel/Slitherlink/index.htm got some puzzles up to 45x31. You can sort them by size (Größe) or difficulty (Schwierigkeitsgrad).
http://www.nikoli.com/en/puzzles/slitherlink/ got some hard ones, but only a few free samples.
https://www.chiark.greenend.org.uk/~sgtatham/puzzles/js/loopy.html can generate puzzles of any size, limited only by your computer's power (and your patience). It also has different types of grids, which can be more difficult.
Here are three sites where you can find very difficult puzzles:
If you want to start with the most difficult, try an "Unlimited" at the first site.