I always use "Budget" they are a reputable international company with competitive rates on a variety of vehicles.
If it is your first time you may want to think twice about driving. I certainly would not consider driving in or around Bangkok. Thailand has one of the worlds worst road safety records and the traffic in Bangkok is insane..
May be take the train or fly up north to Chiang Mai and hire a car there where the roads are quieter.
Only drink bottled water and the street food is fine.

