Out of all of the Thai islands/beaches we visited, Koh Phangan seemed to have the most authentic and affordable food.

• Phatchara Boutique Hotel [Thong Sala/Town]: The people at our hotel were very nice, offering us fresh juice at arrival and even upgrading our room without us asking. So, we overlooked the fact that their prices were higher than average for the area and had breakfast here. I had vegetables with rice, in a delicious sauce, while B had crepes once and curry once. Tasty, though somewhat pricey.
• Night Market [Thong Sala/Town]: I liked eating here best because all four of us (me, B, my cousin, and her bf) could order different foods and still eat together. We tried a few of the roti, which were like fried crepes with sweet fillings – and then sweetened, condensed milk added on top! We also had a few Thai green papaya salads (som tam), which were very good and quite affordable. An older woman had a stand with different types of curries that you could taste with a giant spoon, making your own composite plate from your favorites. There were also stands selling fruit, meats to be fried, Western desserts, and even pizza. There are plenty of cafeteria-style tables to sit at, and the market is open until midnight or so. Recommended!
• Place on Taladmai Road [Thong Sala/Town]: We stumbled on this place our first night, purely because it was still open when we attempted to get dinner. It turned out to be excellent, so we returned twice more – once with my cousin and her boyfriend. We tried a few different types of curry, a fish dish, a chicken dish, a mango salad, and more: we didn’t have one bad dish here. The cook seems to be the owner, and his wife and a few other women help out. There is a cooler with beer, water, and soda. There are also plenty of tables, and they often have the TV on in the corner, showing a Thai soap opera. The desk at which you pay is in the back, next to the TV, as is the table on which the owners’ toddler often sleeps in the evenings. One time, I was standing next to B as he paid; the little girl had woken up and had decided to watch the soap from her spot. Well, I turned out to be blocking her view, which she expressed by scrunching her little hands into fists and giving me a dirty look, all without saying a sound. As soon as I realized what she was about, I moved immediately. However, that didn’t lessen the cuteness of her reaction. Highly recommended! 
• High Life Bungalow [Haad Yao]: We had an okay meal at the place my cousin and her boyfriend were staying. The ambiance was nice, overlooking the beach – and the service was solid. I can’t recall exactly what we ordered: apparently, it didn’t leave much of an impression on me.
• Place next door to the Coconut Beach Bungalows [Coconut Beach]: We ate dinner on the beach, gobbling up their last mango sticky rice after having some curries and fish. The location overlooking the beach was nice, and there weren’t many people there competing for the manager’s attention.
• Place next to entrance to Phaeng Waterfall [Center of Island]: B and I stopped here on our way to the waterfall, having not realized how ravenous we were. He got a hot soup, and I ordered a curry. Both were decent, and not as expensive as we would have expected for the remote, tourist-attraction-adjacent location. They were burning wood nearby when we came, and all the seats were outdoors, so it was a bit pungent at the time; I’m not sure if this is a regular occurrence.