It depends on how much you are transferring.

I've always used bank transfers and SWIFT, and if large amounts telephone the receiving bank and see if you can get a better rate. Actually with Stan Chart they telephone me to let me know it's coming and I always ask for a better rate.

For THB always send in foreign currency and get the onshore THB rate in Thailand by getting your receiving bank to convert into THB. If your sending bank converts into THB for you, the rate isn't as good.

If you are a Stan Chart Priority banking customer they also waive the transaction fee, if you transfer from one Stan Chart acc in one country to another.

On smaller amounts, companies like Transferwise can be a better deal than banks sometimes, as they often don't charge a transaction fee. They make their money by offering a slightly worse rate/ wider spread 

So if you're doing say GBP 100 and your bank charges GBP 10 transaction fee (effectively 10%) + a better rate via SWIFT, you'd likely be better off with the FX co. of no fee and worse rate.

On the other hand for say GBP 100,000 a GBP 10 transaction fee is peanuts in % terms and you're probably better with the bank who gives a better rate

As I say though for me, as Stan Chart don't charge a transaction fee in the first place, it's always the best option.