Valid Characters in a Password?

Hello,
TL;DR
as my account was recently raided and I lost almost everything I decided to change my password to something long and secure. However I don´t seem to get a long password with special characters to work. As in a different post here in the forum was mentioned WOTLK only accepts 16 characters I tested only shorter ones even though the website accepts 18 characters ( 1234567890abcdefgh works for example).

I theoretically want the most secure password possible.

My old password was 8 characters and contained a $ and it worked.

A newly generated password with 16 characters and a $ at the end does not work.

A 16 character password containing { or ' does not work

My question now is:
What are the allowed characters and other limitations for passwords?

To be honest, I would not care about making your password super weird, but rather how you store it afterwards. Do not use public computers, insecure networks, and make sure your computer is free from keyloggers or other sort of spy-mal-ware.
A simple 8 characters password would take 8^26 tries by just using alphabet password, that is an entire life considering the delay between the server response and their machine. If you add in numbers to it it makes even more impossible. However, do not use generics password (such as 1234, batman or similar).