I hadn't heard about walletgenerator.net, so the chances for me to trust it are nearly zero. If you really want to go with the paper wallet way, I'd recommend you downloading the source code of bitaddress.org somewhere locally and then use it offline. As ranochigo wrote, during the generation it's confirmed that it calls the CSPRNG function, which is a known one. Keep in mind that bitaddress.org has been used in the past by a lot of people and their source code has been studied. I wouldn't be that sure for walletgenerator.net.
do you know the best way to create a secure paper wallet?
May I ask you why you want the paper wallet method? There are tons of reasons why you should avoid it, but the most important one is that you're having 1 address per wallet. You can achieve the same thing with
electrum, by storing a twelve-words seed phrase instead of a private key and you'll have access to as many addresses as you want.
Cons of paper wallet:- Only one address.
- You write down the entire private key, which may result into possible mistakes.
(Example of a paper wallet you'll have to print and write the proper values)
Pros of seed phrase:- Unlimited addresses.
- You only write 12 words.
(Example of a seed phrase written on paper)
(By importing them to electrum)