What is the best visa option for visiting Thailand for long stays?

No visa will let you stay as long as you like. They all have limits.

Visa exempt will get you 75 days inside of Thailand before you have to do a border run.

A single entry tourist visa will get you 90 days inside of Thailand before you have to do a border run.

A multiple entry visa can get you around 9 months but you will have to do at least 2 border runs. In order to get a multiple entry tourist visa, you will need to prove your need during the application process. Usually showing a flight into Thailand, a flight out of Thailand, and a flight back into Thailand at least.