I've had good experiences of Budget (4x) and Avis (1x). I'd pay the extra 100 baht a day or so for Collision Damage Waiver. But even if you don't, your liability in the case of a scrape is limited. You take the car away full of fuel and bring it back full of fuel. No messing about charging you for a full tank.

Obviously better to pre-book so it's waiting for you. Avis is BA's Car Rental Partner. I pay about £600 for 22 days with a Vios 1.5 manual, which is a small (but not too small) 5 seater saloon. Driving in Thailand you never stop looking in your mirrors. Video of driving in Bangkok here: www.honestjohn.co.uk/news/miscellaneous/2013-09/dr.../