Even though I live in Thailand, I'm not a huge Thai food fan so, as much as possible, I eat western food. Luckily for me, and for you if you're the same, there are many restaurants in Bangkok that serve western food. Of course, I have my favorite western restaurants, those that I think offer the best western food in Bangkok.
Au Bon Pain - An American chain, Au Bon Pain now has branches all over Bangkok. Serving awesome sandwiches, soup in bread bowls, bagels, salads, wraps and, of course, tons of pastries, cookies and cheesecakes, Au Bon Pain is an amazing place for a quick breakfast or a delicious lunch or dinner. Prices are good too.They open as early as 6:30am (one of the few Bangkok restaurants that do) and close around 10pm. Can't recommend them enough. Siam Discovery, SBC Plaza, Siam Paragon and Siam Square all have Au Bon Pain branches.
Crepes and Company - One of my favorite more upscale western restaurants in Bangkok is Crepes and Company. They serve almost 100 different kinds of crepes (savory and sweet) with lots of choices of stuffings, and every one I've had there is delicious. They also have moussaka, salads, soups, pastas and more. The desserts are gorgeous too. Crepes and Company is located in a converted traditional Thai house and is just lovely and, if it's a cooler day, why not sit out in the pretty garden? Located on Sukhumvit, Soi 12 in Bangkok.
Bourbon Street - Given tons of awards as the best restaurant in Bangkok, Bourbon Street serves Cajun and Mexican food, with some Thai food thrown in. Famous for their Crab Cakes, Beef Brisket and Burritos, you'll leave Bourbon Street completely stuffed, but it's oh so worth it. On Sukhumvit, Soi 22.
There are thousands of restaurants in Bangkok that serve western food, but these three, I think, are the best of the best. They're so good, I dream about their food in my sleep -really.