I have to leave thailand for a new tourist visa. I will take a flight in order to get the longest possible visa. can anyone suggest the best neighboring country to do it? i.e. longer visa, short process?

will appreciate any advise.

All tourist visas as the same so no need to fly. Visa exempt entry by land would be 15 and or by air 30 days and a tourist visa entry will be 60 days for most people regardless of how they travel. In Laos and Malaysia you apply in morning and receive next workday afternoon. In Cambodia it may take a day or two longer at times.