He did the most for Thailand.
1. He revolutionalised the country by putting an end to "slavery".
2. He negotiated Thailand out of becoming France and UK's colony. (Hence, why Thai people are so proud of never having been colonized). The strategy was very clever.
Other than these significant two, he also did a number of things, namely...
Bringing some of the western systems into Thailand;
- The introduction/use of banknote and coins
- New town management system (provinces and towns)
- First railway was built
The abandonment of slavery was a revolution. Slavery was very big before his time. People were bought and sold and the class system was to be passed on to the offspring. It was very cruel.