My favorite is Massaman curry but it varies quite a lot between places, can be too sweet in some. I found a great one in the Emporium Food Court a few years ago but the past few times I've looked I couldn't find it. Best one I did find on my last trip was at Lengkee in Pattaya. They also do great duck dishes there as well, Duck with Pineapple was a revelation, did not expect it to taste so nice.
Massaman at Lengkee
Duck & Pineapple at Lengkee
The Green Chicken Curry at Sanguan Sri on Wireless Road in Bangkok is one of the best Thai dishes I've had. Found out about it on Mark Wiens blog (https://www.eatingthaifood.com/sanguan-sri-restaurant/), not far from Nana so I had a walk round. Only open 10am - 2pm and mainly full of office workers. The sauce was amazing, the chicken is on the bone but really tender. Keep going back to try it when I'm in Bangkok
Green Curry from Sanguan Sri