Bangkok University is OK. It's not the best, but it's certainly not the worst. I visited a class and everyone seemed nice. The lecture seemed straight forward. I'm assuming you'll be in the international program; this means you'll be on the central campus which is a short walk from Ekkamai skytrain station. It's a decent centralized location. Assumption University is split into two campuses: Huamark and Bangna--both locations are horribly inconvenient for international students.
The best university in Thailand is Chulalongkorn. It also has the best location (right behind Siam Square). They have international programs, but they are a bit more expensive than Bangkok University and Assumption University. The down side is, if you go to CU, you might have to spend more time studying due to the higher-level of academics.
Finally, I'll urge you to stay far away from Assumption University. Granted, it is referred to as Thailand's first international university; it is by far the worst university a foreigner could attend. I took 8 courses there: only 2 were taught by professors with a semi-intelligible working knowledge of English. The remaining lecturers used broken English mixed with Thai. Moreover, the registrar, a jackass named Kamol Kitsawad, is a complete dick to ALL international students. And because of his unprofessional and unethical behavior, we almost sued the university. The situation was resolved before it got that far, but that doesn't negate what happened. Avoid ABAC like the plague.
Go to BU.