Bangkok is full of very nice hotels that belong to the western chains and independents or Asian chains that are on par with those kinds of hotels. They've got to all compete with each other. So don't be afraid to branch out beyond Marriott and Hilton unless collecting points and nights are important to you. As for closest to the action, I think everybody has covered the options pretty well. But if you decide to branch out farther (like Sukhumvit or even the cluster of options at Ratchadamri there are many very nice hotels at reasonable prices
I haven't been to Pattaya in a while, but there the standard wasn't quite as high last time I was.