AIS "marathon" SIM, ~1500 THB/year, advertised 15 Mbps speed and 100GB traffic/month
tested: average speed is 10Mbps, maximum speed was ~30 Mbps, checked with servers in Bangkok (speedtest), Japan (testmy.net), and my own servers in France and Netherlands
I believe this "15 Mbps" SIM could yield even higher speeds but I have pretty bad network coverage here (about 20% LTE signal and about 40% 3G signal)
still I would prefer DTAC over AIS but I have a few DTAC SIMs already and thus have to diversify the network connections.