The best and safest is always your own home country. However, if you're already in Thailand and want to travel to the surrounding countries and apply there, I would personally recommend Laos. They will invite you for an interview, but at least you have the opportunity to speak to someone and explain some of the details of your visa application
This Facebook group was created to give you all the support you need. We help each other, and it's better than any self-proclaimed professional visa agent. I read so deeply here that I ended up checking everything five times word for word, letter for letter with my own visa application. But typos aren't everything. It gets funny with "place of issue," for example. Without the help here in the Facebook group, I certainly could have applied for the visa three times.
You said that it was rejected without explanation, but the explanation is that the expiration date of the passport does not match the expiration date in the visa application. There is no chance of getting the money back, that's what happened to many people.
If you're going there, I recommend staying at the hotel directly across from the consulate. If you're invited to the interview or need to go to the consulate to check on the status of your visa application, you can simply cross the street and avoid having to deal with greedy taxi drivers.
There will be an interview in Vientiane and Savannakhet, so most people avoid Laos. If all your documents are authentic and you can verify the amount in your banking app on your phone, then you have nothing to worry