What visa should I apply for if I'm traveling to Thailand for 140 days?

It all depends on what visas you meet the qualifications and requirements for. Going to the official Thai Government WEB site and read the requirements for each one that fits your planned length of stay and then you can figure out which best fits your individual situation.

You're putting the cart in front of the horse. You already have plans to stay for 140 days and later ask about a visa. I sure hope you don't have everything booked as you are probably going to be disappointed. Without knowing anything else about you or your intentions it's very difficult to advise you on a visa which may suit you.

You can either get a 6 month multiple entry tourist visa that allows you to stay 60 days at a time for 6 months

Have you looked at the visa options, why 140 days.

DTV if you meet the requirements

Depends on your age

140 days is tricky

It may require multiple

Exits / re-entries

Need to know

Over 50 ?

Unless you go for a long-term visa like a Non-OA there really isn't one that will give you 140 days without having to leave the country. If you're coming as a tourist you should look at the Multiple Entry Tourist Visa (METV) which is valid for 6 months from the date it's issued, you get stamped in for 60 days on arrival and can extend for an additional 30 days at an immigration office inside Thailand then you would need to leave to a nearby country and return for another 60 days

Age, Thai family, passport etc.?