I'm going to come out in favor of DHL, I had one bad experience with them and I took that up with their HQ in Germany - everything got sorted out and the cost of shipping was refunded in full.
Since then, I found them prompt, fair and responsive. Yes, they can be a bit expensive but if you want something on a certain deadline, they pull the stops out. I always prepay the tax through their web app and everything is smooth as silk.
I'm fairly sure that most of the major couriers in TH are not actual branches of their respective companies; they're agents who are licensed to operate on their behalf. As such, you can expect the same type of shenanigans that get with any other local cowboy.
FedEx have been okay-ish, if a little slow and they tend to punt out their deliveries to third-party drivers for the last leg.
UPS on the other hand are in a different league, the local defamation laws preclude me from going into details but, if you ever get a call from UPS saying there's a problem with your shipment, tell them to send it back.