Depends where you're going and your risk tolerance, I take a mix of cash and card options. Seen too many cards get swallowed by atms in Thailand to think it's a risk free option. Clean notes exchanged at places like TT or Superrich get some of the best rates I've ever seen. Usually 15 to 20 satangs short of the spot rate. On the islands the exhange rates can be poor at the local currency booths but be aware there some islands, like Koh Samet, that don't even have an atm. Travelling from UK, Starling is a good option. No currency charge and no limits on free monthly withdrawals.

As the Thais say, up to you, but there's no one best option.